-505 error in proactive backups

[twickland 69]

When a client has multiple favorite folders used as sources in a proactive script, it appears that two activity threads can attempt to back up two different favorite folders on the same client simultaneously. This generates a -505 error, which results in neither folder being backed up. Retrospect will then not attempt to access the client again until the error wait interval specified in the script has passed. This is probably not a big deal if the proactive script is active all the time, but can cause a source to miss a backup opportunity if the failure occurs too close to a specified script stop time, as we have discovered.