Raise codec errors when casting to std::string (#2903)

* Raise codec errors when casting to std::string

Allow the codec's exception to be raised instead of RuntimeError when
casting from py::str to std::string.

PY2 allows ucs surrogates in UTF-8 conversion

Signed-off-by: Shane Loretz <sloretz@openrobotics.org>
Signed-off-by: Shane Loretz <sloretz@osrfoundation.org>

* Attempt to fix py2 error

* Revert all unicode literals

* Fixed

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>