Handle SNMP exception error.PySnmpError

In ironic.drivers.modules.snmp.py, SNMPClient._get_transport() could
raise an SNMP exception. This is now caught and the ironic exception
SNMPFailure is raised instead.

Change-Id: I8816729b1b9483c213bb9e94b0816a88e609a91d
Partial-Bug: #1267693
Merge "Handle SNMP exception error.PySnmpError"