Change logger from exception to warning

Test suite keeps failing when it handles the logger.exception output.
This change is temporary and probably needs a revisit / better way to
handle it.

embed_video/templatetags/embed_video_tags.py

@@ -122,9 +122,9 @@ def render(self, context):
                 "Timeout reached during rendering embed video (`{0}`)".format(url)
             )
         except UnknownBackendException:
-            logger.exception("Backend wasn't recognised (`{0}`)".format(url))
+            logger.warning("Backend wasn't recognised (`{0}`)".format(url))
         except VideoDoesntExistException:
-            logger.exception("Attempt to render not existing video (`{0}`)".format(url))
+            logger.warning("Attempt to render not existing video (`{0}`)".format(url))
 
         return ""