This removes unnecessary error handling that resulted in a couple of
bugs: One, if a rename operation failed (e.g. the new filename is too
long), the file's name did not get reverted. Two, when the new requested
filename was already in use, it would correctly ask the user for a new
one but after the operation finished successfully the file with the old
filename would be selected instead of the new one.
There's no need to manually check for a filename conflict, KIO will
handle that for us and only proceed with the operation when it should.
To be more consistent with other error messages in Dolphin, the error
message dialog when a rename operation fails was removed and instead the
errorMessage signal will be emitted. Currently this results in the error
message being displayed at the top.