Emit underlying error code when pretty-printing I/O errors (#4121)

Author: neonichu

Sources/Workspace/Diagnostics.swift

@@ -175,8 +175,8 @@ extension FileSystemError: CustomStringConvertible {
             switch self.kind {
             case .invalidAccess:
                 return "invalid access"
-            case .ioError:
-                return "encountered I/O error"
+            case .ioError(let code):
+                return "encountered I/O error (code: \(code))"
             case .isDirectory:
                 return "is a directory"
             case .noEntry:
@@ -199,8 +199,8 @@ extension FileSystemError: CustomStringConvertible {
         switch self.kind {
         case .invalidAccess:
             return "invalid access to \(path)"
-        case .ioError:
-            return "encountered an I/O error while reading \(path)"
+        case .ioError(let code):
+            return "encountered an I/O error (code: \(code)) while reading \(path)"
         case .isDirectory:
             return "\(path) is a directory"
         case .noEntry: