Changes

change default no-proxy reject cause to net-fail

If LU has to be rejected because DGSM mechanism cannot reach the
needed HLR, then returning reject cause #17 (Network Failure)
is the most accurate and truthful description of what is actually
happening.

Reject cause #2 (IMSI unknown in HLR) is unlikely to be a good
choice: in areas where regular commercial cell services are present
alongside with experimental/adventurous Osmocom-based GSM networks,
this reject cause is acutely dangerous as described in the previous
commit, and even in areas where no regular commercial cell services
are present (the presumed intended use case for DGSM), the behavior
of commanding LU-failing phones "please don't ever again try to
connect to ANY network until the user power-cycles you" is unlikely
to be correct/desirable.  If that behavior _is_ desired, it should
be configured explicitly, not by default.

Change-Id: I557ae1d3291066a87228b5db4f2e207ea721329c