Raise RuntimeError when MeTTa extension is loaded with error

[vsbogd]

This is fast fix to raise an error when erroneous extension is loaded:

from hyperon.ext import register_atoms

@register_atoms
def my_atoms():
    return {
        'multi-sum': NonExistingAtom()
    }

Right now error is too verbose because the whole stack is added in hyperonpy.cpp. To avoid it and return only message sent by user we need to introduce custom exception class to be able to distinguish it from other RuntimeErrors.