fix: print error when internal cli error happens

There have been issues where the reported error is "ERROR: null", which
is very unhelpful. Here is an analysis of a specific example:
#5048 (comment)

Although the trigger for that error was fixed by #5074, the
unhelpfulness of "ERROR: null" was not addressed.

To help with debugging, this patch prints the original error when this
stage is unexpectedly reached.

Author: Rob--W

lib/cli/cli.js

@@ -66,6 +66,11 @@ exports.main = (argv = process.argv.slice(2), mochaArgs) => {
       debug("caught error sometime before command handler: %O", err);
       yargs.showHelp();
       console.error(`\n${symbols.error} ${pc.red("ERROR:")} ${msg}`);
+      if (!msg) {
+        // Log raw error and stack when an unexpected error is encountered, to
+        // make debugging easier (instead of an inactionable "ERROR: null").
+        console.error(err);
+      }
       process.exit(1);
     })
     .help("help", "Show usage information & exit")