libpq-oauth: Don't translate internal errors

libpq-oauth: Don't translate internal errors

Some error messages are generated when OAuth multiplexer operations fail
unexpectedly in the client. Álvaro pointed out that these are both
difficult to translate idiomatically (as they use internal terminology
heavily) and of dubious translation value to end users (since they're
going to need to get developer help anyway). The response parsing engine
has a similar issue.

Remove these from the translation files by introducing internal variants
of actx_error() and oauth_parse_set_error().

Suggested-by: Álvaro Herrera <[email protected]>
Reviewed-by: Álvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/CAOYmi%2BkQQ8vpRcoSrA5EQ98Wa3G6jFj1yRHs6mh1V7ohkTC7JA%40mail.gmail.com
Backpatch-through: 18