The error code as retrieved by raw1394_get_errcode is converted into a
roughly equivalent errno number and returned. 0xdead is returned for an
illegal errcode.
It is intended to be used to decide what to do (retry, give up, report error)
for those programs that aren't interested in details, since these get lost in
the conversion. However the returned errnos are equivalent in source code
meaning only, the associated text of e.g. perror is not necessarily
meaningful.