accounts.py: Use ui.error when raising exceptions

Rather than using ui.warn, use ui.error() which outputs Exceptions to
the error log, saving them to a stack, so we get notified again at the
end of the sync run.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

Changelog.draft.rst

@@ -13,6 +13,10 @@ others.
 New Features
 ------------
 
+* When a message upload/download fails, we do not abort the whole folder
+  synchronization, but only skip that message, informing the user at the
+  end of the sync run.
+ 
 Changes
 -------