Improve error message for misuse of TZ, tz, OF formatting patterns.

Be specific about which pattern is being complained of, and avoid saying
"it's not supported in to_date", which is just confusing if the error is
actually coming out of to_timestamp.  We can phrase it as "is only
supported in to_char", instead.  Also, use the term "formatting field" not
"format pattern", because other error messages in the same file prefer that
terminology.  (This isn't terribly consistent with the documentation, so
maybe we should change all these error messages?)