Improve error reporting on Exceptions in fw views.

When there's an exception during the inflation of a framework view (for
example invalid attributes), report the exception correctly. The earlier
behaviour assumed the exception to be a ClassNotFoundException and tried
to load it from the user's project. This is not longer the case.

Also, update the MockView class to a FrameLayout with a single TextView.
This means that the MockView is a ViewGroup and will not choke when
someone attempts to add a View to it (although, the view will be
silently dropped).

Change-Id: Ice003817ceb627ebfbbbb245ab6be10f9141e728