Stop build if there were build-tool plugin failures

Sources/Build/BuildOperation.swift

@@ -405,6 +405,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         )
 
         // Surface any diagnostics from build tool plugins.
+        var succeeded = true
         for (target, results) in buildToolPluginInvocationResults {
             // There is one result for each plugin that gets applied to a target.
             for result in results {
@@ -420,9 +421,14 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
                 for diag in result.diagnostics {
                     diagnosticsEmitter.emit(diag)
                 }
+                succeeded = succeeded && result.succeeded
             }
         }
 
+        if !succeeded {
+            throw StringError("build stopped due to build-tool plugin failures")
+        }
+
         // Run any prebuild commands provided by build tool plugins. Any failure stops the build.
         let prebuildCommandResults = try graph.reachableTargets.reduce(into: [:], { partial, target in
             partial[target] = try buildToolPluginInvocationResults[target].map { try self.runPrebuildCommands(for: $0) }

Tests/CommandsTests/PackageToolTests.swift

@@ -1408,7 +1408,7 @@ final class PackageToolTests: CommandsTestCase {
             XCTAssertNotEqual(result.exitStatus, .terminated(code: 0), "output: \(output)")
             XCTAssertMatch(output, .contains("This is text from the plugin"))
             XCTAssertMatch(output, .contains("error: This is an error from the plugin"))
-            XCTAssertMatch(output, .contains("Build complete!"))
+            XCTAssertMatch(output, .contains("build stopped due to build-tool plugin failures"))
         }
     }