Fix tryCatch error handling

[rbchan]

When hessian can't be inverted, it fails instead of returning NA.

[kenkellner]

I have been working on this a little bit. Assuming you still want to change this, is the desired behavior when the Hessian can't be inverted to throw a warning and return a matrix of NAs?

I was thinking of adding a common function that all fitting functions would use for this, for example:

invertHessian <- function(optimOut, nparam, SE){

  blankMat <- matrix(NA, nparam, nparam)
  if(!SE) return(blankMat)

  tryCatch(solve(optimOut$hessian),
    error=function(e){
      warning("Hessian is singular. Try providing starting values or using fewer covariates.", call.=FALSE)
      return(blankMat)
  })
}

[rbchan]

That looks great, thanks.