Show the original error if calling director method failed

[vadz]

Previously, if calling the director method failed, the precise error
message was lost and just "Error detected" was shown to the user, making
it difficult to understand what the problem actually was.

Change the code to show the contents of the last Python error too to
make finding and fixing errors in Python director methods much simpler.

[ojwb]

How likely are these checks to fail? We could avoid converting msg to std::string and back when we're just going to use it unaltered.

[vadz]

AFAIK they can never fail at all, the checks are just there to account for the "impossible" failures.

[ojwb]

OK, then there's no point complicating things.

It'd be good to have a testcase covering this change, but otherwise it looks sensible to me.

[letmaik]

See my comment at #1117 (comment). I don't think this PR is the right approach.

[degasus]

I have the opposite opinion of letmaik here. There are valid reasons why some exceptions must be handled in pure C++ (e.g. async worker threads calling broken python code), however I want to ask if the error message fetching should be done in the what() method instead. This removes the overhead of forwarded exceptions, and at the same time, it provides a way for pure C++ handlers to understand the issue.

[ojwb]

In #1117 wsfulton said:

I agree that we do need a better default and the default should be just to [throw] the original exception. The behaviour should be fully customisable too, for example it should be possible to not have any C++ exceptions as requested in #315.

Based on that I'm closing this as not the preferred approach.

@degasus said above:

There are valid reasons why some exceptions must be handled in pure C++ (e.g. async worker threads calling broken python code), however I want to ask if the error message fetching should be done in the what() method instead. This removes the overhead of forwarded exceptions, and at the same time, it provides a way for pure C++ handlers to understand the issue.

I think you'd need to provide a custom director:except typemap if you want that.