Report errors when saving masks to *_seg.npy

[deroulers]

Currently, in the GUI, when saving the masks in a *_seg.npy file fails, no error message is printed and the process silently stops in the call to np.save in gui/io.py, function _save_sets.

With this PR, an error message would be displayed, giving hints about the cause of the failure (e.g. ``permission denied''), and the process would go further (e.g. updating the graphical display if needed, which should happen even if the call to np.save failed).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.69%. Comparing base (4d30972) to head (f976bd1).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1129   +/-   ##
=======================================
  Coverage   52.69%   52.69%           
=======================================
  Files          18       18           
  Lines        4342     4342           
=======================================
  Hits         2288     2288           
  Misses       2054     2054           

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[carsen-stringer]

thanks!