This PR was squashed before being merged into the 2.0.x-dev branch (closes #953).

Discussion
----------

Improve exception messages for mount()

Improve the exception message if the `connect` method of a `ControllerProviderInterface` returns an invalid type.

Previously a misleading message about the arguments to `mount()` was emitted if your `connect` method did not return a `ControllerCollection`.

Commits
-------

7d6298d8 Improve exception messages for mount()