Remove inner begin/rescue.

This rescue unnecessarily caught all exceptions. This particularly
masked the ShutdownSignal exception, which would prevent Logstash from
shutting down properly.

This rescue isn't necessary; there is already a rescue around the
function call, so this one is being removed.
snmptrap: handle shutdown

When trying to shutdown logstash, the snmptrap plugin would endlessly
try to restart. This change makes the plugin respond to the
ShutdownSignal exception by killing the listening thread and returning.
This allows Logstash to shutdown normally.
Remove extra begin/rescue nesting.

Methods have an implicit begin/end within them; there is no need to add
an extra one.