[mlir:python] Prevent crash in DenseElementsAttr. (llvm#163564)

This PR fixes a crash in the `bf_getbuffer` implementation of
`PyDenseElementsAttribute` that occurred when an element type was not
supported, such as `bf16`. I believe that supportion `bf16` is not
possible with that protocol but that's out of the scope of this PR.
Previsouly, the code raised an `std::exception` out of `bf_getbuffer`
that nanobind does not catch (see also pybind/pybind11#3336). The PR
makes the function catch all `std::exception`s and manually raises a
Python exception instead.

Signed-off-by: Ingo Müller <[email protected]>

Author: ingomueller-net

mlir/lib/Bindings/Python/IRAttributes.cpp

@@ -1306,6 +1306,10 @@ PyType_Slot PyDenseElementsAttribute::slots[] = {
     e.restore();
     nb::chain_error(PyExc_BufferError, "Error converting attribute to buffer");
     return -1;
+  } catch (std::exception &e) {
+    nb::chain_error(PyExc_BufferError,
+                    "Error converting attribute to buffer: %s", e.what());
+    return -1;
   }
   view->obj = obj;
   view->ndim = 1;