Expose object name error fields in PL/pgSQL.

Specifically, permit attaching them to the error in RAISE and retrieving
them from a caught error in GET STACKED DIAGNOSTICS.  RAISE enforces
nothing about the content of the fields; for its purposes, they are just
additional string fields.  Consequently, clarify in the protocol and
libpq documentation that the usual relationships between error fields,
like a schema name appearing wherever a table name appears, are not
universal.  This freedom has other applications; consider a FDW
propagating an error from an RDBMS having no schema support.

Back-patch to 9.3, where core support for the error fields was
introduced.  This prevents the confusion of having a release where libpq
exposes the fields and PL/pgSQL does not.

Pavel Stehule, lexical revisions by Noah Misch.