Convert unknow code (out of spec) of NERP to 99
Based on Patrick's observation:
[...] when parsing NERP NegativeReason. If the code is unknown (outside of the given list in the RFC), the current code throws an Exception. This patch does not fail but log a warn and set the reason to an Unspecified Reason (99). I believe that we should not completely fail for such a small error from the remote side. Otherwise we never know that we got a NERP. [...]