improve error handling of availability check (#6506)

motvation: more verbose error message when availability check fails

changes: wrap availability check error handling with explicit error

rdar://108418554

Author: tomerd

Sources/PackageRegistry/RegistryClient.swift

@@ -361,12 +361,13 @@ public final class RegistryClient: Cancellable {
                                         }
 
                                         SignatureValidation.extractSigningEntity(
-                                        signature: [UInt8](signatureData),
-                                        signatureFormat: signatureFormat,
-                                        configuration: configuration,
-                                        fileSystem: fileSystem,
-                                        completion: wrappedCompletion
-                                    ) }
+                                            signature: [UInt8](signatureData),
+                                            signatureFormat: signatureFormat,
+                                            configuration: configuration,
+                                            fileSystem: fileSystem,
+                                            completion: wrappedCompletion
+                                        )
+                                    }
                                 }
                             )
                         },
@@ -1534,25 +1535,26 @@ public final class RegistryClient: Cancellable {
         let start = DispatchTime.now()
         observabilityScope.emit(info: "checking availability of \(registry.url) using \(request.url)")
         self.httpClient.execute(request, observabilityScope: observabilityScope, progress: nil) { result in
-            completion(
-                result.tryMap { response in
-                    observabilityScope
-                        .emit(
-                            debug: "server response for \(request.url): \(response.statusCode) in \(start.distance(to: .now()).descriptionInSeconds)"
-                        )
-                    switch response.statusCode {
-                    case 200:
-                        return .available
-                    case let value where AvailabilityStatus.unavailableStatusCodes.contains(value):
-                        return .unavailable
-                    default:
-                        if let error = try? response.parseError(decoder: self.jsonDecoder) {
-                            return .error(error.detail)
-                        }
-                        return .error("unknown server error (\(response.statusCode))")
+            switch result {
+            case .success(let response):
+                observabilityScope
+                    .emit(
+                        debug: "server response for \(request.url): \(response.statusCode) in \(start.distance(to: .now()).descriptionInSeconds)"
+                    )
+                switch response.statusCode {
+                case 200:
+                    return completion(.success(.available))
+                case let value where AvailabilityStatus.unavailableStatusCodes.contains(value):
+                    return completion(.success(.unavailable))
+                default:
+                    if let error = try? response.parseError(decoder: self.jsonDecoder) {
+                        return completion(.success(.error(error.detail)))
                     }
+                    return completion(.success(.error("unknown server error (\(response.statusCode))")))
                 }
-            )
+            case .failure(let error):
+                return completion(.failure(RegistryError.availabilityCheckFailed(registry: registry, error: error)))
+            }
         }
     }
 
@@ -1671,6 +1673,7 @@ public enum RegistryError: Error, CustomStringConvertible {
     case unauthorized
     case authenticationMethodNotSupported
     case forbidden
+    case availabilityCheckFailed(registry: Registry, error: Error)
     case registryNotAvailable(Registry)
     case packageNotFound
     case packageVersionNotFound
@@ -1770,6 +1773,8 @@ public enum RegistryError: Error, CustomStringConvertible {
             return "authentication method not supported"
         case .forbidden:
             return "forbidden"
+        case .availabilityCheckFailed(let registry, let error):
+            return "failed checking availability of registry at '\(registry.url)': \(error.interpolationDescription)"
         case .registryNotAvailable(let registry):
             return "registry at '\(registry.url)' is not available at this time, please try again later"
         case .packageNotFound:
@@ -1870,7 +1875,13 @@ extension RegistryClient {
             public let signing: Signing?
             public let signingEntity: SigningEntity?
 
-            public init(name: String, type: String, checksum: String?, signing: Signing?, signingEntity: SigningEntity?) {
+            public init(
+                name: String,
+                type: String,
+                checksum: String?,
+                signing: Signing?,
+                signingEntity: SigningEntity?
+            ) {
                 self.name = name
                 self.type = type
                 self.checksum = checksum