If a python model is loaded and initialized, but is missing some dependencies, an unspecified exception is raised

[hellkite500]

ngen/include/realizations/catchment/Bmi_Py_Adapter.hpp

Line 646 in 07454f7

bmi_model->attr("initialize")(bmi_init_config);

Traced an error down to initializing a module, and inside that module, a python error No module named ... is thrown. So initialize fails, but the only error ngen produces is

terminate called after throwing an instance of 'std::exception'
  what():  std::exception

This should produce some sort of meaning error/message to the user about the issues loading/initializing the python module.

[mattw-nws]

For the record, did this exist before #468 and b7ae8fc?