Catch errors if the model name isn't found

Author: penelopeysm

ad.py

@@ -103,7 +103,11 @@ def run_ad(args):
 
 def get_model_definition(model_key):
     """Get the model definition from the file that contains it."""
-    return Path(f"models/{model_key}.jl").read_text().strip()
+    try:
+        return Path(f"models/{model_key}.jl").read_text().strip()
+    except FileNotFoundError:
+        return (f"Model definition not found for `{model_key}`;
+                f" if you see this error, please open an issue.")
 
 
 def html(_args):