Mouse Cursor stays on loading/processing state if an error is produced on the processing

[RafaelPalomar]

Describe the bug

"Loading/Computing/Busy" cursor doe not change state back to normal when an error is produced in an operation.

This happens in a couple of situations at least:

• "Add/Modify Segment Centerline"
• "Compute Distance Maps"

To Reproduce

• Add/Modify Segment Centerline: Try to compute a centerline without having added any centerline end-points
• Compute Distance Maps: Try to compute the distance maps with two labelmaps which don't share the same space.

Expected behavior

• Error is produced and the cursor goes back to normal

Desktop (please complete the following information):

• At least Linux, but this should affect all OSs