Raise exception on HTTP errors in backend. Closes #34.

bigbluebutton/api/bigbluebutton.py

@@ -37,6 +37,11 @@ except ImportError:  # pragma: no cover
 logger = logging.getLogger(__name__)
 
 
+class BigBlueButtonError(Exception):
+    """Exception raised when a BigBlueButton backends encounters an error."""
+    pass
+
+
 @dataclass
 class BigBlueButton:
     """One BigBlueButton server.
@@ -134,6 +139,9 @@ class BigBlueButton:
         url = self._build_url(call, params)
 
         res = self._session.get(url, timeout=self.request_timeout)
+        if res.status_code != 200:
+            raise BigBlueButtonError(f"Backend returned HTTP status {res.status_code}.")
+
         xml = xmltodict.parse(res.text)
 
         return xml["response"]