pgsql: Make src/common/exec.c's error logging less ugly.

Make src/common/exec.c's error logging less ugly.

This code used elog where it really ought to use ereport, mainly so that
it can report a SQLSTATE different from ERRCODE_INTERNAL_ERROR. There
were some other random deviations from typical error report practice too.

In addition, we can make some cleanups that were impractical six months
ago:

* Use one variadic macro, instead of several with different numbers
of arguments, reducing the temptation to force-fit messages into
particular numbers of arguments;

* Use %m, even in the frontend case, simplifying the code.

Discussion: https://postgr.es/m/[email protected]

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b6b297d20df9f738be20a450f80bade535819220

Modified Files
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src/common/exec.c | 124 +++++++++++++++++++++++++++++++-----------------------
1 file changed, 72 insertions(+), 52 deletions(-)