Clean up dubious error handling in wellformed_xml().

This ancient bit of code was summarily trapping any ereport longjmp
whatsoever and assuming that it must represent an invalid-XML report.
It's not really appropriate to handle OOM-like situations that way:
maybe the input is valid or maybe not, but we couldn't find out.
And it'd be a seriously bad idea to ignore, say, a query cancel
error that way.  (Perhaps that can't happen because there is no
CHECK_FOR_INTERRUPTS anywhere within xml_parse, but even if that's
true today it's obviously a very fragile assumption.)

But in the wake of the previous commit, we can drop the PG_TRY
here altogether, and use the soft error mechanism to catch only
the kinds of errors that are legitimate to treat as invalid-XML.

(This is our first use of the soft error mechanism for something
not directly related to a datatype input function.  It won't be
the last.)

xml_is_document can be converted in the same way.  That one is
not actively broken, because it was checking specifically for
ERRCODE_INVALID_XML_DOCUMENT rather than trapping everything;
but the code is still shorter and probably faster this way.

Discussion: https://postgr.es/m/3564577.1671142683@sss.pgh.pa.us

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