Add getters for exception type, value and traceback

[martinRenou]

In order to retrieve the exception type, value and trace, we can use sys.exc_info() in Python. But using pybind11 it's not that easy currently.
When an exception occurs in Python code it's directly converted to py::error_already_set and it holds the error. Which means that if we want to retrieve the type, value and trace we first need to restore the error using the .restore() method, and then call PyErr_Fetch, which is not ideal.
I added accessors to objects describing the error in py::error_already_set.

Please tell me what you think :)

[YannickJadoul]

@martinRenou Shouldn't/couldn't this be const object &get_type() const { return type; }, as C++-style to avoid refcounting in the copy constructor when it's not necessary to copy the object object?

[YannickJadoul]

@martinRenou Sorry to bother you again, but before I can rebase and force-push #1651, the methods also need to be const (so I can call it on the caught const py::error_already_set & in a catch block).

[martinRenou]

Don't worry :) I'll do it. Do you have an idea why the build is not passing?

[YannickJadoul]

I'm looking into that now, actually. On Travis, it's got something to do with a dead link, it seems. Nothing in this PR, I'd guess :-)

[YannickJadoul]

Although ... the current build failure is also partly due to this PR, I think, since I'm getting error messages on "duplicate const".
Probably this const object& get_type() { return type; } const instead of const object& get_type() const { return type; } typo?

[martinRenou]

Oh yeah sorry... I did that too quickly with vim without really checking what I was doing

[YannickJadoul]

Great, thanks! :-)
Rebased #1651 onto this, already.

About the failing tests: #1642 is fixing the broken build and solving the Travis part.

[zhihaoy]

Unless someone whats to bikeshed it, I think this is good, because I've started to use it (I thought about letting vcpkg to patch it, at least in our team's vcpkg clone).

[martinRenou]

Any chance to merge this?

[wjakob]

This seems reasonable. However, just a comment regarding the naming: we haven't so far used get_ as prefix for getters in this library.

[wjakob]

actually, scratch that :) -- there are quite a few prefixed get_.

[martinRenou]

I can change it to type, value and trace if you prefer :)

[wjakob]

Hm, I think I would prefer them to be named just value(), trace(), etc. (you will probably have to change the name of the members then).

There is one slight inconsistency with handle::get_type which actually does use a get_ prefix, but elsewhere we generally omit it.

[martinRenou]

@wjakob Hope it's fine now :)

[martinRenou]

Any chance to get this in? @wjakob. I guess the travis job not passing is unrelated.

[apollo13]

I am running with this patch locally, works fine so far

[wjakob]

Merged, thanks!

[martinRenou]

Thanks :)