Add assertion string to references and document_symbol requests to language server

These endpoints could return `None` in case the symbol is one that's not
found. Adding the assertion string can help caller catch these
exceptions and handle the errors as appropriate.

Author: jetzhou

src/multilspy/language_server.py

@@ -467,7 +467,7 @@ async def request_references(
             )
 
         ret: List[multilspy_types.Location] = []
-        assert isinstance(response, list)
+        assert isinstance(response, list), f"Unexpected response from Language Server: {response}"
         for item in response:
             assert isinstance(item, dict)
             assert LSPConstants.URI in item
@@ -597,7 +597,7 @@ async def request_document_symbols(self, relative_file_path: str) -> Tuple[List[
 
         ret: List[multilspy_types.UnifiedSymbolInformation] = []
         l_tree = None
-        assert isinstance(response, list)
+        assert isinstance(response, list), f"Unexpected response from Language Server: {response}"
         for item in response:
             assert isinstance(item, dict)
             assert LSPConstants.NAME in item