Improve reporting of invalid weight symbols in setweight() et al.

Improve reporting of invalid weight symbols in setweight() et al.

This commit addresses two related issues:

tsvector_filter() assumed it could print an incorrect weight value
with %c.  This could result in an invalidly-encoded error message
if the database encoding is multibyte and the char value has its
high bit set.  Weight values that are ASCII control characters
could render illegibly too.  Fix by printing such values in octal
(\ooo), similarly to how charout() would render them.

tsvector_setweight() and tsvector_setweight_by_filter() reported
the same unrecognized-weight error condition with elog(), as though
it were an internal error.  That'd not translate, would produce an
unwanted XX000 SQLSTATE code, and also reported the bad value as a
decimal integer which seems unhelpful.  Fix by refactoring so that
all three functions share one copy of the code that interprets a
weight argument.

The invalid-encoding aspect seems to me (tgl) to justify
back-patching.

Author: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAON2xHNaeLAUzRCXL5AmXLcXaSE_gWAVjWQRmLzc_oZ=1_Vf4Q@mail.gmail.com
Backpatch-through: 14