Fix confusing dialog when logging out with an on-going reencryption

[vxgmichel]

A Cancelled exception was showed in a dialog with the confusing message:

Reencryption failed for unknown reason

when a user logs out with an on-going reencryption.

[codecov]

Codecov Report

Merging #1663 (d77b2a4) into master (1add871) will increase coverage by 21.22%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           master    #1663       +/-   ##
===========================================
+ Coverage   66.49%   87.71%   +21.22%     
===========================================
  Files         243      243               
  Lines       23407    23409        +2     
===========================================
+ Hits        15564    20533     +4969     
+ Misses       7843     2876     -4967     

Impacted Files	Coverage Δ
parsec/core/gui/workspaces_widget.py	83.54% <50.00%> (+59.81%)	⬆️
parsec/backend/postgresql/block.py	88.40% <0.00%> (-1.45%)	⬇️
parsec/backend/invite.py	89.36% <0.00%> (+0.28%)	⬆️
parsec/backend/postgresql/invite.py	96.42% <0.00%> (+0.59%)	⬆️
parsec/core/fs/userfs/userfs.py	85.96% <0.00%> (+0.64%)	⬆️
parsec/backend/organization.py	98.62% <0.00%> (+0.68%)	⬆️
parsec/core/local_device.py	93.38% <0.00%> (+0.73%)	⬆️
parsec/core/fs/exceptions.py	98.47% <0.00%> (+0.76%)	⬆️
parsec/core/mountpoint/fuse_runner.py	96.72% <0.00%> (+0.81%)	⬆️
parsec/core/fs/workspacefs/versioning_helpers.py	98.71% <0.00%> (+0.85%)	⬆️
... and 63 more

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