python API crashing with C++ exceptions

Created by: siebenkopf

The color conversion functions in the Python API, such as bob.ip.color.rgb_to_gray might crash with a C++ exception, which is not caught correctly inside the python bindings.

Solution: go through the functions and add a BOB_TRY ... BOB_CATCH_FUNCTION wrapper around them (see http://pythonhosted.org/bob.extension/cpp_api.html#c.BOB_CATCH_FUNCTION).