pgconn: add OnError to Config for error handling

[jameshartig]

OnError is called on every error response received from Postgres and can be used to close connections on specific errors. Defaults to closing on FATAL-severity errors.

Fixes #1803

[jackc]

LGTM with one possible change. Perhaps it could be made more explicit that this is only for PostgreSQL protocol level errors and not for network or any other kind of errors. Maybe more docs on the OnError field or maybe to include PG in the new identifiers (OnPGError, PGErrorHandler, etc.)

[jameshartig]

@jackc good point, I just updated that to clarify and added PG to the fields/types.

[jackc]

I merged it. I ended up tweaking the names a bit in df3c5f4 to use Pg instead of PG. I had forgotten that the error type is actually PgError not PGError and figured it is better to be consistent.

[jameshartig]

I merged it. I ended up tweaking the names a bit in df3c5f4 to use Pg instead of PG. I had forgotten that the error type is actually PgError not PGError and figured it is better to be consistent.

Good call! Apologies I should've noticed that.