Minor edits.
Fix bug where n0 gradually crept upwards.

* simImage.realize was using the current n0 from the model for
  generation.  For multiple simulations, model.n0 was being set
  in the likelihood map loop, so it gradually crept upwards until
  the code crashed.
* Improve error message when bracketing fails.
* This bug should not have affected any previous results. It
  1) Only applied to multiple simulation when the likelihood was mapped.
  2) Crashed the code pretty quickly in that case (after 6 simulations).