WTAP_ERR_UNWRITABLE_ errors aren't returned by reads or open-for-reading.

Check for them *only* on opening for writing and writes.

Change-Id: I4b537d511ec04bcfc81f69166a2b9a2ee9310067
Reviewed-on: https://code.wireshark.org/review/5827
Reviewed-by: Guy Harris <guy@alum.mit.edu>