fits: fixed FITS write error message for >999 columns

FITS tables cannot support more than 999 columns, since 8-character
table metadata keywords like TTYPEnnn can only accommodate 3-digit
column indices.  Previously, this failure appeared as a "Keyword too
long" error generated within the FITS library, which wasn't very
easy for users to understand.  I've fixed this so it's now clear
from the error message what's wrong.

When this first came up (June 2012) I considered fiddling with the
header cards to write something like TTYPnnnn.  I aired this suggestion
on the fitsbits mailing list where it was not well-received.
I think the only way to get round it in vanilla FITS would be to
break the table up into 999-column chunks and put them in successive
HDUs, which would be bad for streaming as well as making it complicated
to store multiple tables in the same FITS file.