Add btwxt error handling

[chipbarnaby]

HPWHsim uses btwxt for some interpolation tasks.

However, it appears that there is no support for possible btwxt errors. The following should be considered and probably added --

1. Exception handling. Based on quick review of btwxt code, it can throw. try / catches probably needed around some calls?
2. courierr error callback scheme needed to capture and forward error messages.

Test case(s) should be added to verify the above work correctly.