fix(cli/rt/error_stack): Improve message line formatting

[nayeemrmn]

Matches browsers.

Example:

const error = new Error();
error.name = undefined;
error.message = undefined;
console.log(error.stack);

// Before:
// undefined: undefined
//     at ...

// After:
// Error
//     at ...

[bartlomieju]

Great! Thanks @nayeemrmn