Add highlighting to error messages (#331)

This is done by using the `::error::{message}` [workflow command
syntax][1].

Fixes #330

[1]: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-error-message

internal/github/action.go

@@ -63,7 +63,7 @@ func (a *Action) CheckLabels(stdout, stderr io.Writer) int {
 func (a *Action) handleFailure() int {
 	a.outputResult("failure")
 	err := errors.New(a.trimTrailingNewLine(a.failMsg))
-	fmt.Fprintln(a.Stderr, "Error:", err)
+	fmt.Fprintln(a.Stderr, "::error::", err)
 	if a.allowFailure() {
 		return 0
 	}

internal/github/action_test.go

@@ -114,7 +114,7 @@ func TestLabelChecks(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			tc.expectedStdout = "Checking GitHub labels ...\n" + tc.expectedStdout
 			if len(tc.expectedStderr) > 0 {
-				tc.expectedStderr = "Error: " + tc.expectedStderr
+				tc.expectedStderr = "::error:: " + tc.expectedStderr
 			}
 			setPullRequestNumber(tc.prNumber)
 			tc.specifyChecks()