Detect when a slot is aborted and gracefully exit

Following lb-nightly-builds-frontend!135 (merged), I added checks in a few specific places so that if the slot was flagged as aborted we stop gracefully the operations.

In particular I cannot stop the checkout or build of a project, but I prevent the next one to start and if the abort arrives during the processing of last project the job completes normally.

Accessory tasks are not aborted (copy of ccache caches or local installation to prepare the tests).

See LBCORE-1968