Change array's writeable exception to a ValueError

Numpy raises ValueError when attempting to modify an array, while
py::array is raising a RuntimeError.  This changes the exception to a
std::domain_error, which gets mapped to the expected ValueError in
python.

include/pybind11/numpy.h

@@ -536,7 +536,7 @@ protected:
 
 
     void check_writeable() const {
     void check_writeable() const {
         if (!writeable())
         if (!writeable())
-            throw std::runtime_error("array is not writeable");
+            throw std::domain_error("array is not writeable");
     }
     }
 
 
     static std::vector<size_t> default_strides(const std::vector<size_t>& shape, size_t itemsize) {
     static std::vector<size_t> default_strides(const std::vector<size_t>& shape, size_t itemsize) {

tests/test_numpy_array.py

@@ -92,7 +92,7 @@ def test_mutate_readonly(arr):
     from pybind11_tests.array import mutate_data, mutate_data_t, mutate_at_t
     from pybind11_tests.array import mutate_data, mutate_data_t, mutate_at_t
     arr.flags.writeable = False
     arr.flags.writeable = False
     for func, args in (mutate_data, ()), (mutate_data_t, ()), (mutate_at_t, (0, 0)):
     for func, args in (mutate_data, ()), (mutate_data_t, ()), (mutate_at_t, (0, 0)):
-        with pytest.raises(RuntimeError) as excinfo:
+        with pytest.raises(ValueError) as excinfo:
             func(arr, *args)
             func(arr, *args)
         assert str(excinfo.value) == 'array is not writeable'
         assert str(excinfo.value) == 'array is not writeable'