You may not always know which schemes are available.
The library should not bail out, but only show
an informal message. It's the responsibility of
the application to deal with invalid URI schemes.
The test brings a Finder window to the front. It's not ideal,
but I have no better idea at the moment. It would be cool if we
can make the test case register itself as handler for a particular
uri scheme, but I have no idea how to do that.
