Catch accessing boards before wait start

Author: WillB97

sbot/internal/board_manager.py

@@ -417,6 +417,9 @@ def find_output(self, identifier: str, idx: int) -> OutputIdentifier:
         :raises KeyError: If no board with the given identifier is registered.
         """
         assert idx >= 0, "Output identifiers must be positive"
+        assert self.loaded, \
+            "Boards cannot be accessed before they are loaded. Run utils.load_boards() first."
+
         try:
             return self.outputs[identifier][idx]
         except IndexError: