Convert xml_in to report errors softly.

Convert xml_in to report errors softly.

The key idea here is that xml_parse must distinguish hard errors
from soft errors.  We want to throw a hard error for libxml
initialization failures: those might be out-of-memory, or something
else, but in any case they are not the fault of the input string.
If we get to the point of parsing the input, and something goes
wrong, we can fairly consider that to mean bad input.

One thing that arguably does mean bad input, but I didn't trouble
to handle softly, is encoding conversion failure while converting
the server encoding to UTF8.  This might be something to improve
later, but it seems like a pretty low-probability scenario.

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