SNMP function read and write doesn't detect correctly that PacketFence is not able to connect

[fdurand]

Describe the bug
Define a cisco 2960 in packetfence and forget to configure snmp.
The code doesn't detect that snmp is not available and try to do a snmp read and produce a rest timeout.

[nqb]

@jrouzierinverse, I think you made some code to handle this issue. Is it ready?

[satkunas]

Adjust timeouts, better error handling.