Improve some error message strings and errcodes

Improve some error message strings and errcodes

This makes a bit less work for translators, by unifying error strings a
bit more with what the rest of the code does, this time for three error
strings in autoprewarm and one in base backup code.

After some code review of slot.c, some file-access errcodes are reported
but lead to an incorrect internal error, while corrupted data makes the
most sense, similarly to the previous work done in e41d0a1.  Also,
after calling rmtree(), a WARNING gets reported, which is a duplicate of
what the internal call report, so make the code more consistent with all
other code paths calling this function.

Author: Michael Paquier
Discussion: https://postgr.es/m/20180902200747.GC1343@paquier.xyz