`MR::Model::InvalidError`, set backtrace to caller of `save`

This properly sets the backtrace for the `InvalidError` exception
that an `MR::Model` throws to the caller of `save`. Previously, it
would return a backtrace to MR internals, which is not ideal or
useful when using MR. This points it at the caller of `save` which
is what should be expected when using MR.

Closes #144