bsnmpget should print protocol errors to stderr

[eugen_grosbein.net]

Summary

Currently it is hard to determine if remote SNMP agent supports needed OID/subtree using automated tools with bsnmpget(1) because it prints an error to stdout mixing it with received other data.

This change makes it print error to stderr, as well as bsnmpwalk/bsnmpset that is same tool really.

Test Plan

Try to check for IF-MIB::ifHCInMulticastPkts

bsnmpwalk -o quiet -n -r 3 -t 2 -s public@switch .1.3.6.1.2.1.31.1.1.1.8 > /dev/null

[bz]

Looks good to me; let's wait if syrinx thinks the same.

[syrinx]

The change looks reasonable to me, please commit