tests (unit/cli/calls): Skip tests in case of missing media or webrtc:

Test were raising exceptions when optional modules were missing (GStreamer related modules
for `webrtc`). Those are now catched so the tests are simply skipped in this case.

`MissingModule` is now inheriting from `ImportError`, as it's a natural subclass, and it
makes import error catching easier.

libervia/backend/core/exceptions.py

@@ -54,7 +54,7 @@ class UnknownGroupError(Exception):
     pass
 
 
-class MissingModule(Exception):
+class MissingModule(ImportError):
     # Used to indicate when a plugin dependence is not found
     # it's nice to indicate when to find the dependence in argument string
     pass

tests/unit/cli/test_calls.py

@@ -20,11 +20,16 @@ from unittest.mock import MagicMock, patch
 import pytest
 
 try:
+    from libervia import media
+    import webrtc
     from libervia.cli.call_gui import AVCallUI
     from libervia.frontends.tools import display_servers
     from PyQt6.QtWidgets import QApplication
-except ModuleNotFoundError:
-    pytest.skip("PyQt6 is not available.", allow_module_level=True)
+except ImportError as e:
+    pytest.skip(
+        f"Required dependencies not available ({type(e).__name__}: {e}).",
+        allow_module_level=True,
+    )
 
 
 @pytest.fixture(scope="session")