process: cast promise rejection reason to string

The unhandled promise rejection warning uses a template literal and
prints the reason a promise was rejected. If rejecting with a symbol,
the symbol failed to convert to a string and the process crashed. Now,
symbols are casted to strings and the process does not crash.

Fixes: https://github.com/nodejs/node/issues/11637
PR-URL: https://github.com/nodejs/node/pull/11640


Reviewed-By: Anna Henningsen <anna@addaleax.net>