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

src/backend/utils/adt/int8.c

@@ -121,8 +121,8 @@ invalid_syntax:
 	if (!errorOK)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
-				 errmsg("invalid input syntax for integer: \"%s\"",
-						str)));
+				 errmsg("invalid input syntax for type %s: \"%s\"",
+						"bigint", str)));
 	return false;
 }