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