Clarify failure in snmp_output(..) with call to snmp_pdu_decode

• Explicitly test snmp_pdu_encode against SNMP_CODE_OK instead of assuming any non-zero value is bad.
• Print out the code before calling abort() to give the end-user something actionable to debug without having to recompile the binary, since the core might not have these details.

MFC after: 1 week