Fix recognition of missing XDG user directories

[codesoap]

Description:

In dialog/file_xdg.go there was some code to check, if a user directory does not exist. This code did not work for me, because locURI.String() was, for example, file:///home/richard/ when the "Videos" directory wasn't found. homeUri.Strin() was file:///home/richard.

I am not sure if xdg-user-dir always outputs the trailing slash, so I took a conservative approach and used strings.TrimRight.

I've added a second commit, which prevents the errors from getFavoriteLocation to be overwritten by errors from storage.ListerForURI.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 64.751% (+0.02%) from 64.729%
when pulling 63aa9e0 on codesoap:xdg_favorites
into ca0e9e5 on fyne-io:develop.

[andydotxyz]

Thanks