improve ControlMaster error reporting (gravitational#30631)

integration/integration_test.go

@@ -4463,15 +4463,12 @@ func testExternalClient(t *testing.T, suite *integrationTestSuite) {
 			if tt.outError {
 				require.Error(t, err)
 			} else {
-				if err != nil {
-					// If an *exec.ExitError is returned, parse it and return stderr. If this
-					// is not done then c.Assert will just print a byte array for the error.
-					er, ok := err.(*exec.ExitError)
-					if ok {
-						t.Fatalf("Unexpected error: %v", string(er.Stderr))
-					}
+				// ensure stderr is printed as a string rather than bytes
+				var stderr string
+				if e, ok := err.(*exec.ExitError); ok {
+					stderr = string(e.Stderr)
 				}
-				require.NoError(t, err)
+				require.NoError(t, err, "stderr=%q", stderr)
 				require.Equal(t, tt.outExecOutput, strings.TrimSpace(string(output)))
 			}
 		})