reword error message when import is failing due to errors in resources already present

[lsteinmann]

sometimes, an import can fail due to an invalid resource that will be changed / affected by the import though the file that is being imported has no issues. in that case, it is not clear for the user that the file is fine and they need to solve the issue with the affected resource(s) first. the warning system makes this easy to do, which is why a more helpful error message would be enough here.