Merge pull request #2579 from szabgab/ask_the_preprocessor_to_blow_up

[test] Check content of error message

tests/custom_preprocessors.rs

@@ -43,6 +43,18 @@ fn ask_the_preprocessor_to_blow_up() {
     let got = md.build();
 
     assert!(got.is_err());
+    let error_message = got.err().unwrap().to_string();
+    let status = if cfg!(windows) {
+        "exit code: 1"
+    } else {
+        "exit status: 1"
+    };
+    assert_eq!(
+        error_message,
+        format!(
+            r#"The "nop-preprocessor" preprocessor exited unsuccessfully with {status} status"#
+        )
+    );
 }
 
 #[test]