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/6025.1527351693@sss.pgh.pa.us