skip_repaired_exemplar without exemplar

If the option skip_repaired_exemplar is True, BluePyMM still checks if the exemplar exists and try to evaluate them.
For example:
https://github.com/BlueBrain/BluePyMM/blob/master/bluepymm/prepare_combos/create_mm_sqlite.py#L115
this check does not pass, but I guess it wants to be here: https://github.com/BlueBrain/BluePyMM/blob/master/bluepymm/prepare_combos/create_mm_sqlite.py#L119

For evaluations, it just raises an exception, so it should be ok for now.