Add missing test to testsuite

Ref. #511
Merge pull request #528 from JohannesPfeifer/master

Fix typo in model_diagnostics.m
Octave compatibility fix: catch does not accept an argument
Fix test under Octave, by changing the value of mode_compute

With the former optimizer, the ML estimation under Octave would lead to a point
with a non-positive covariance matrix for shocks; in turn this would be an
unsuitable starting point for the subsequent Bayesian estimation
Fix error message when initial values of estimation are not suitable.

Ref #512
Fix bug when datafile was changed and nobs resulting from old dataset was bigger than is possible in new dataset