Improve error message for failed steps

Author: swissspidy

src/Context/Support.php

@@ -123,6 +123,23 @@ protected function check_string( $output, $expected, $action, $message = false,
 				$message = $output;
 			}
 
+			$action_message = '';
+			switch ( $action ) {
+				case 'be':
+					$action_message = 'Output does not exactly match expected string:';
+					break;
+				case 'contain':
+					$action_message = 'Output does not contain expected string:';
+					break;
+				case 'not contain':
+					$action_message = 'Output unexpectedly contains string:';
+					break;
+			}
+
+			if ( ! empty( $action_message ) ) {
+				$message .= "\n\n" . $action_message . "\n" . $expected;
+			}
+
 			$diff = $this->generate_diff( $expected, rtrim( $output, "\n" ) );
 			if ( ! empty( $diff ) ) {
 				$message .= "\n\n" . $diff;

src/Context/WhenStepDefinitions.php

@@ -19,7 +19,27 @@ public function wpcli_tests_invoke_proc( $proc, $mode ) {
 		);
 		$method = $map[ $mode ];
 
-		return $proc->$method();
+		try {
+			return $proc->$method();
+		} catch ( \RuntimeException $e ) {
+			if ( 'run' === $mode ) {
+				$message = $e->getMessage();
+				$status  = 'unknown';
+				if ( preg_match( '/exit status: (\d+)$/', $message, $matches ) ) {
+					$status = $matches[1];
+				}
+
+				if ( '0' === $status ) {
+					$message .= "\n\nThe command unexpectedly produced STDERR output. If you expect a non-zero exit status or STDERR output, use `When I try [...]`. Else, this may be a bug in your code or test.";
+				} else {
+					$message .= "\n\nThe command unexpectedly exited with status {$status}. If you expect a non-zero exit status, use `When I try [...]`. Else, this may be a bug in your code or test.";
+				}
+
+				throw new \RuntimeException( $message, $e->getCode(), $e );
+			}
+
+			throw $e;
+		}
 	}
 
 	/**