Error reporting when dot is not available

[tudorache]

Protege 5.1.0 and newer report the missing dot in the logs and console (much better than in the dialog). There seem to be a lot of the same error logged for the same session, which clogs the logs.

Ideally, the error should be reported once (maybe when the plugin initializes and/or the UI refreshes) rather than each time OWLViz is invoked.