DismanPing updates: don't return stale data, log more helpful errors, and rename weird variable

1. If SNMP stops repsonding, etc., we will return stale data because we didn't clear out the rtts.  Clear them at the beginning of ping() like others do.

2. While debugging point 1, found a weird variable reuse.  Perl scoping rules made it OK but it is still confusing.

3. Parse the SNMP_Session::errmsg to try to give users more helpful error messages.