gui/main: Send only one notification per error (#1776)
gui/main: Send only one notification per error (#1776)

To avoid spamming the user with the same error notification each time
they open the main app window, we keep track of the last notified
message and only send a system notification if the error message is
new.
cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop,cozy-labs/cozy-desktop