Handle C++ exceptions gracefully

[kwabenantim]

Summary

When a wrapped C++ function throws an exception that pybind11 doesn't know how to translate, the exception may propagate out of the binding, hit std::terminate, and crash the whole Python process instead of getting translated into a catchable Python exception. Although pybind11 auto-translates std::exception and its subclasses, exception classes in some C++ code do not derive from std::exception and throw types pybind11 may not be able to handle.

Currently, users can work around this with manual try/except blocks on the Python side. However, it would be good if this didn't have to be done manually.

pybind11 provides a py::register_exception_translator(...) that can be used to catch arbitrary C++ exception types and raise a corresponding Python exception.

[kwabenantim]

Caveat

The deeper issue here seems to be that pybind11::register_exception_translator can only catch C++ exceptions.

C error return codes (such as PetscErrorCode) are ignored and can result in an abort (e.g. by PetscAbortErrorHandler) which terminates the process. The C error will need be turned into a C++ exception in the main C++ code before pybind11 can see it (e.g. by using PetscCallThrow(), which throws a std::runtime_error()).