Hang when invalid file format is specified

[waveform80]

When a format which has a valid MIME type but no associated encoder is specified, picamera hangs.

[waveform80]

This occurs because the encoder lock in the camera class is non-re-entrant; the camera class acquires this lock when starting a capture / recording but once the encoder class encounters the unsupported encoding it raises an exception. In doing so, it calls the stop() method which attempts to re-acquire the lock within the same thread and the hang occurs.

This could be fixed by moved the lock acquisition down a line in the stop() method but I'm not sure if this introduces a race condition ... needs more testing.