Improve logging of 'Tape session finished' message

Problem

As discussed in https://gitlab.cern.ch/cta/operations/-/issues/1289, the TAS Tape Alerting System has no way to know if a reported tape session failure happened due to an unexpected error or due to a normal workflow event (which should not trigger an alert).

Some examples of workflow events that do not trigger a DCS alarm:

• Empty mount.
• Retrieve queue is sleeping.
• Tape transfer process killed by an operator.
• ...

Objective

As proposed in https://gitlab.cern.ch/cta/operations/-/issues/1289, add a failureReason field to the "Tape session finished" message, so that the TAS system can easily identify which failures warrant an alarm:

Example:

• Empty mount:
  • failureReason set to tapeNotMounted emptyQueue
• Retrieve queue is sleeping:
  • failureReason set to sleepingRetrieveQueue
• Tape transfer process killed by an operator.
  • failureReason set to operatorTerminated
• Actual errors that warrant an alarm:
  • failureReason set to unnexpected ??