Changes

s1ap_proxy: catch exceptions in handle_pdu/2

We do have a try-catch block in handle_pdu_bin/2 that does catch
exceptions happening in decode_pdu/1, but does not catch anything
else.  For instance, if something goes wrong in handle_pdu/2,
the whole process will be terminated, among with all the linked
E-RAB FSMs.  This should definitely be avoided.

Change-Id: Ib1e6674f5b85557866c7beaea710ea903e4eeaca