file plugin: starting an already started element is a bug in the base class

This MR was initially about returning true when start was invoked on an already started element, but it turned out that this would actually be a bug in the base class. See the discussion below.

Here is the initial MR description for the record:

Attempting to start an already started filesrc or filesink should not return in error as this would cause the base class to post an ErrorMessage on the bus. According to GstMessage's documentation, in the event of an ErrorMessage, the application "should stop playback of the pipeline and not assume that more data will be played".