NXCALS-1568 check if keytab exists

#Merge request template @acc-logging-team - please have a look for approval.

Adding checks and custom exceptions in case of the absence of the keytab file.

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For some reason sonarcube added its (completely irrelevant) comments from my previous pull request to this one. Is there a way to have this removed?

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Closes NXCALS-1568