Changes

m3ua: Fix fsm dispatch failure not generating tx ERR msg to peer

Recent commit reworked those code paths to return an error type instead
of an err message, but wrongly left the negative error of
osmo_fsm_inst_dispatch() code path as is, without converting it to a
positive m3ua error code value, which ended up in an ERR msg not being
generated.

Fixes: 82c1d2c8c6699349258869bce690293b590a6b0d
Change-Id: I16042c97453854c78e55bfe6bd6925506eae82c5