Improve failure reporting (#104)

In certain cases, the exception message reported by Gradle was
insufficient or incorrect.
This PR improves a couple of things:

1. Propagate failure received in `FinishEvent`, rather than reporting
incorrectly (#103)
2. Use a contextual exception for missing input parameters, to ensure
that the underlying cause is correctly reported.

plugin-test/src/test/groovy/org/gradle/github/dependencygraph/DependencyExtractorConfigTest.groovy

@@ -96,6 +96,6 @@ class DependencyExtractorConfigTest extends BaseExtractorTest {
  def result = executer.runWithFailure()
 
  then:
- result.output.contains("'GITHUB_DEPENDENCY_GRAPH_JOB_CORRELATOR' must be set")
+ result.output.contains("> The configuration parameter 'GITHUB_DEPENDENCY_GRAPH_JOB_CORRELATOR' must be set")
  }
 }

plugin/src/main/kotlin/org/gradle/dependencygraph/extractor/DependencyExtractor.kt

@@ -12,6 +12,7 @@ import org.gradle.dependencygraph.model.*
 import org.gradle.dependencygraph.util.*
 import org.gradle.initialization.EvaluateSettingsBuildOperationType
 import org.gradle.initialization.LoadProjectsBuildOperationType
+import org.gradle.internal.exceptions.Contextual
 import org.gradle.internal.exceptions.DefaultMultiCauseException
 import org.gradle.internal.operations.*
 import java.io.File
@@ -100,14 +101,16 @@ abstract class DependencyExtractor :
  val details: D? = buildOperation.details.let {
  if (it is D) it else null
  }
- val result: R? = finishEvent.result.let {
- if (it is R) it else null
+ if (details == null) {
+ return // Ignore other build operation types
  }
- if (details == null && result == null) {
-  return
- } else if (details == null || result == null) {
- throw IllegalStateException("buildOperation.details & finishedEvent.result were unexpected types")
+
+ val failure = finishEvent.failure
+ if (failure != null) {
+ throw failure
  }
+
+ val result: R = finishEvent.result as R
  handler(details, result)
  }
 
@@ -311,15 +314,11 @@ abstract class DependencyExtractor :
  try {
  writeDependencyGraph()
  } catch (e: RuntimeException) {
- throw GradleException(
- "The dependency-graph extractor plugin encountered errors while writing the dependency snapshot json file. " +
- "Please report this issue at: https://github.com/gradle/github-dependency-graph-gradle-plugin/issues",
- e
- )
+ throw DefaultMultiCauseException("Failed to write dependency-graph to file", e)
  }
  }
 
  companion object {
  private val LOGGER = Logging.getLogger(DependencyExtractor::class.java)
  }
-}
+}