Don't show error message on ERR_USER_CANCELED

Merged Kai Uwe Broulik requested to merge work/kbroulik/dont-error-on-user-canceled into master

There's no point telling the user what they just did.

After frameworks/kio@f192c133 this error has a text associated with it, so it might show up in the UI.

I think this should go into stable

@ahmadsamir @felixernst

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