Pipeline never finishes when the acquisition fails

When an acquisition fails (aka, an exception is thrown in the acquisition loop) the processing is never properly finished (is_finished stays true).

Two options:

• Cancel the current pipeline
• Send an empty frame (tagged as is is_final) to process all the data that were acquired before the failure. This would require to adapt every processing node to properly handle this empty frame though.

We might also want to pass the error information to the on_end_acq callback, together with nb_frames_acquired.