test(promise): test for error object names instead of specific messages

[glennsl]

textExternalPromiseThrow failed for me in node v19.6 because the exception message had changed from

Unexpected token . in JSON at position 1

to

Expected property name or '}' in JSON at position 1

It's rarely a good idea to test for specific string messages unless you control them in the test itself, since they're solely intended for humans and changing them typically isn't considered breaking.

[zth]

Good catch! Just want to make the minimum needed changes.

[glennsl]

I rewrote the tests to actually test what they're supposed to instead of a fragile side-effect. Is that better?

[zth]

Sure, let's go with this.