Deduplicate "invalid input syntax" messages for various types.

Deduplicate "invalid input syntax" messages for various types.

Previously a lot of the error messages referenced the type in the
error message itself. That requires that the message is translated
separately for each type.

Note that currently a few smallint cases continue to reference the
integer, rather than smallint, type. A later patch will create a
separate routine for 16bit input.

Author: Andres Freund
Discussion: https://postgr.es/m/20180707200158.wpqkd7rjr4jxq5g7@alap3.anarazel.de