[Feature] abort unarchive job

[pulsargranular]

Description
When user clicks on a (large) dataset to unarchive, it will take quite some time until the data has been read from tape, transferred to disk, verified and unpacked. If the user suddenly decides to stop this process (because he found out that he chose the wrong dataest, for example), this should be possible.

Definition of Done

• User starts unarchival process
• at various stages in the process, an abort can be made
• the process should then stop the process gracefully
• existing copied datablocks should be removed
• status of dataset should be reset back to normal
• proper feedback should be sent to the user