osbuild-worker: improve error "reason" in case of stage failures

cmd/osbuild-worker/jobimpl-osbuild.go

@@ -542,6 +542,7 @@ func (impl *OSBuildJobImpl) Run(job worker.Job) error {
 		return err
 	}
 
+	var failedStage string
 	// Include pipeline stages output inside the worker's logs.
 	// Order pipelines based on PipelineNames from job
 	for _, pipelineName := range osbuildJobResult.PipelineNames.All() {
@@ -556,6 +557,7 @@ func (impl *OSBuildJobImpl) Run(job worker.Job) error {
 				logWithId.Infof("  %s success", stageResult.Type)
 			} else {
 				logWithId.Infof("  %s failure:", stageResult.Type)
+				failedStage = stageResult.Type
 				stageOutput := strings.Split(stageResult.Output, "\n")
 				for _, line := range stageOutput {
 					logWithId.Infof("    %s", line)
@@ -575,7 +577,12 @@ func (impl *OSBuildJobImpl) Run(job worker.Job) error {
 				osbErrors = append(osbErrors, fmt.Sprintf("manifest validation error: %v", err))
 			}
 		}
-		osbuildJobResult.JobError = clienterrors.WorkerClientError(clienterrors.ErrorBuildJob, "osbuild build failed", osbErrors)
+
+		reason := "osbuild build failed"
+		if len(failedStage) > 0 {
+			reason += " in stage:\n" + failedStage
+		}
+		osbuildJobResult.JobError = clienterrors.WorkerClientError(clienterrors.ErrorBuildJob, reason, osbErrors)
 		return nil
 	}