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

@@ -61,6 +61,7 @@ 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}`);
+      console.error(err);
       process.exit(1);
     })
     .help('help', 'Show usage information & exit')