frontends (tools/webrtc): Better message on Python override missing:

raise an exception instead of simple `print` to stop loading the module, and show a more
detailed message with the package to install.

libervia/frontends/tools/webrtc.py

@@ -23,12 +23,14 @@ gi.require_versions({
 })
 from gi.repository import Gst, GstWebRTC, GstSdp
 
+from libervia.backend.core import exceptions
+
 try:
     from gi.overrides import Gst as _
 except ImportError:
-    print(
-        "no GStreamer python overrides available, please install relevant pacakges on "
-        "your system."
+    raise exceptions.MissingModule(
+        "No GStreamer Python overrides available. Please install relevant packages on "
+        "your system (e.g., `python3-gst-1.0` on Debian and derivatives)."
     )
 import asyncio
 from dataclasses import dataclass
@@ -38,7 +40,6 @@ import re
 from typing import Callable
 from urllib.parse import quote_plus
 
-from libervia.backend.core import exceptions
 from libervia.backend.tools.common import data_format
 from libervia.frontends.tools import aio