No need for WTAP_ERR_CANT_READ.

Unlike the standard I/O routines, the code we introduced that supports
fast random seeking on gzipped files will always supply some specific
error code for read errors, so we don't need WTAP_ERR_CANT_READ.

Add WTAP_ERR_CANT_WRITE for writing, as we're still using the standard
I/O routines for that.  Set errno to WTAP_ERR_CANT_WRITE before calling
fwrite() in wtap_dump_file_write(), so that it's used if fwrite() fails
without setting errno.

Change-Id: I6bf066a6838284a532737aa65fd0c9bb3639ad63
Reviewed-on: https://code.wireshark.org/review/4540
Reviewed-by: Guy Harris <guy@alum.mit.edu>