Improved error reporting in format()

The current error message for an invalid format conversion type is
extremely confusing except in the simplest of uses:

select format( '% moo');
ERROR:  unrecognized conversion type specifier " "

Obviously in that example you can figure out what's going on, but
frequently format() is used in a complex context where it's not at all
obvious that format is the problem. Even worse, "conversion" makes it
sound like a localization issue.

Attached patch clarifies that %-related error messages with hints as
well as (IMHO) improving the clarity of the message:

select format( '% moo');
ERROR:  unrecognized format() type specifier " "
HINT:  For a single "%" use "%%"

I also made the use of "format()" consistent in all the other error
messages.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com