High: CTDB: do not fail monitor operation when ctdb socket does not exist

When starting CTDB with a custom socket location for the first
time, pacemaker's probe operation (monitor_0) fails because
the custom socket does not exist. This patch detects this failure
and returns "not running" instead of a "generic error" return code.
Merge pull request #397 from davidvossel/ctdb_fixes

High: CTDB: do not fail monitor operation when ctdb socket does not exist