Neaten up our choices of SQLSTATEs for XML-related errors.

Neaten up our choices of SQLSTATEs for XML-related errors.

When our XML-handling modules were first written, the SQL standard
lacked any error codes that were particularly intended for XML
error conditions.  Unsurprisingly, this led to some rather random
choices of errcodes in those modules.  Now the standard has a whole
SQLSTATE class, "Class 10 - XQuery Error", with a reasonably large
selection of relevant-looking errcodes.

In this patch I've chosen one fairly generic code defined by the
standard, 10608 = invalid_argument_for_xquery, and used it where
it seemed appropriate.  I've also made an effort to replace
ERRCODE_INTERNAL_ERROR everywhere it was not clearly reporting
a coding problem; in particular, many of the existing uses look
like they can fairly be reported as ERRCODE_OUT_OF_MEMORY.

It might be interesting to try to map libxml2's error codes into
the standard's new collection, but I've not undertaken that here.

Discussion: https://postgr.es/m/417250.1726341268@sss.pgh.pa.us