format-parser: Improve error message when format specifier lacks width.

format-parser: Improve error message when format specifier lacks width.

Until now, parse_format_specifier() was willing to parse, e.g. "datetime"
as a format specifier.  It would then be rejected later by e.g.
fmt_check_input() with a message like "Input format DATETIME0.0 specifies
width 0, but DATETIME requires a width between 17 and 40," which makes no
sense to the user.  This commit improves the error message.