Better error report

[HDembinski]

This adds better error messages when py::cast is not used correctly.

[HDembinski]

This job merely had a timeout, could someone with privileges please restart it?
https://travis-ci.org/pybind/pybind11/jobs/601280214

[wjakob]

I would consider an error message like that, but only if compiled in debug mode. (Analogous to other uses of fancy string manipulation throughout the library, which include a note of the sort " (please recompile in debug mode for more details)". This is to avoid code bloat.)

[wjakob]

build restarted.

[HDembinski]

Ok, makes sense. My solution is still not perfect though, because the type name reported is without qualifiers, eq. const T& is reported as T. This is unfortunately how the typeid operator works, it drops all those qualifiers. Unfortunately, the qualifiers are critical to debug issues with the caster.

Keeping the qualifiers will be a more involved patch, which seems better suited for a separate PR.

[HDembinski]

This version now only prints the type name in a debug build.

[wjakob]

Looks good, thanks! Regarding getting the extra flags: I think the easiest and most cross-platform way is to wrap the type T into some dummy template foo<T>, stringify that, and then strip of the outer parts.