pgsql: Make some sanity-check elogs more verbose

Make some sanity-check elogs more verbose

A few sanity checks in funcapi.c were not mentioning all the possible
clauses for failure, confusing developers who fat-fingered catalog data
additions.  Make the errors more detailed to avoid wasting time in
pinpointing mistakes.

Per complaint from Craig Ringer.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAMsr+YH7Kd87A3cU5m_wKo46HPQ46zFv5wesFNL0YWxkGhGv3g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fe051291550ab88267ce3104e9833925bf757393

Modified Files
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src/backend/utils/fmgr/funcapi.c | 25 +++++++++++++++----------
1 file changed, 15 insertions(+), 10 deletions(-)