Fix invalid type error when loading antimony model from file (#2281)

* Fix invalid type error when loading antimony model from file

`pathlib.Path` -> `str`

* More informative exceptions

python/sdist/amici/antimony_import.py

@@ -28,11 +28,13 @@ def antimony2sbml(ant_model: Union[str, Path]) -> str:
         is_file = False
 
     if is_file:
-        status = ant.loadAntimonyFile(ant_model)
+        status = ant.loadAntimonyFile(str(ant_model))
     else:
         status = ant.loadAntimonyString(ant_model)
     if status < 0:
-        raise RuntimeError("Antimony model could not be loaded.")
+        raise RuntimeError(
+            f"Antimony model could not be loaded: {ant.getLastError()}"
+        )
 
     if (main_module_name := ant.getMainModuleName()) is None:
         raise AssertionError("There is no Antimony module.")