Raise ImportError and notify user when specifying a non existant converter graphics backend

apps/converter/api.py

@@ -44,7 +44,10 @@ def _lazy_load(fn):
 def _get_backend():
     return import_module(GRAPHICS_BACKEND)
 
-backend = _get_backend()
+try:    
+    backend = _get_backend()
+except ImportError:
+    raise ImportError('Missing or incorrect converter backend: %s' % GRAPHICS_BACKEND)
 
 def cleanup(filename):
     ''' tries to remove the given filename. Ignores non-existent files '''