Add configuration option to allow plugin to continue walk when errors are encountered

[robbavey]

When encountering an error during an SNMP walk, the plugin will immediately abort the walk, disregarding any data that may previously have been collected.

It is observed that the snmpwalk command line tool when encountering similar errors that cause logstash to fail, will continue the operation and return results it was able to process.

We may wish to provide a similar experience, perhaps gated via a configuration option to keep the current behaviour as the default.