[alias-server] Error response handling patch

Summary:
As observed in D14191, when Cashtab feeds through invalid inputs to the alias-server endpoints, its fetch() calls are returning undefined rather than the expected error object.
Upon review the address and alias endpoints in alias-server's app.js were correctly catching the errors but weren't returning the status 500 error.

Test Plan:
npm test
propagate diff to alias.etokens.cash
retest Cashtab fetch calls in D14191 and D14189

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14193