Handle "I can't map this for that file format" better.

For cases where record (meta)data is something that can't be written out
in a particular file format, return WTAP_ERR_UNWRITABLE_REC_DATA along
with an err_info string.

Report (and free) that err_info string in cases where
WTAP_ERR_UNWRITABLE_REC_DATA is returned.

Clean up some other error reporting cases, and flag with an XXX some
cases where we aren't reporting errors at all, while we're at it.

Change-Id: I91d02093af0d42c24ec4634c2c773b30f3d39ab3
Reviewed-on: https://code.wireshark.org/review/5823
Reviewed-by: Guy Harris <guy@alum.mit.edu>