Improve error message for StringDtype with invalid storage (#55052)

Author: phofl

pandas/core/arrays/string_.py

@@ -123,7 +123,8 @@ def __init__(self, storage=None) -> None:
                 storage = get_option("mode.string_storage")
         if storage not in {"python", "pyarrow", "pyarrow_numpy"}:
             raise ValueError(
-                f"Storage must be 'python' or 'pyarrow'. Got {storage} instead."
+                f"Storage must be 'python', 'pyarrow' or 'pyarrow_numpy'. "
+                f"Got {storage} instead."
             )
         if storage in ("pyarrow", "pyarrow_numpy") and pa_version_under7p0:
             raise ImportError(

pandas/tests/arrays/string_/test_string_arrow.py

@@ -255,3 +255,11 @@ def test_pickle_roundtrip():
 
     result_sliced = pickle.loads(sliced_pickled)
     tm.assert_series_equal(result_sliced, expected_sliced)
+
+
+@skip_if_no_pyarrow
+def test_string_dtype_error_message():
+    # GH#55051
+    msg = "Storage must be 'python', 'pyarrow' or 'pyarrow_numpy'."
+    with pytest.raises(ValueError, match=msg):
+        StringDtype("bla")