Hello,

I am running an estimation using Occbin and mode_compute = 6. Depending on the exact setup of the problem, I occasionally run into a hard error:

```
Error using chol
Matrix must be positive definite.
Error in gmhmaxlik_core (line 194)
dd = transpose(chol(CovJump));
Error in gmhmaxlik (line 100)
[PostMode, PostVariance, Scale, PostMean] = gmhmaxlik_core(fun, OldPostMode, bounds, gmhmaxlikOptions, Scale, flag, MeanPar, OldPostVariance, varargin{:});
```

That is, the minimiser makes it all the way to the “last call”, and then fails due to the jumping matrix not being positive definite. Presumably, however, the matrix must have been positive definite while tuning the scale parameter and so on, so this seemed an unusual moment for an error.

It might be that this is the intended behaviour, but if so could I suggest adding some minimal error handling and a user-friendly error message (like the error messages when the initial likelihood is NaN)?