Implemented optional C++ exceptions in Error class

These can be activated using the -DLAMMPS_EXCEPTIONS compiler flag.
It has no effect for regular execution. However, while using
it as a library, any issued command will capture the exception
and save its error message. This can be queried using the
lammps_has_error() and lammps_get_last_error_message() methods.

The Python wrapper checks these in order to rethrow these errors
as Python exceptions. See issue #146.

(cherry picked from commit 6c154bb0b67a13d38968bc42d31013b97f87db75)