Rename WTAP_ERR_REC_TYPE_UNSUPPORTED to WTAP_ERR_UNWRITABLE_REC_TYPE.

That indicates that it's a problem specific to *writing* capture files;
we've already converted some errors to that style, and added a new one
in that style.

Change-Id: I8268316fd8b1a9e301bf09ae970b4b1fbcb35c9d
Reviewed-on: https://code.wireshark.org/review/5826
Reviewed-by: Guy Harris <guy@alum.mit.edu>