Improve error message for min netcon delay

[pramodk]

Based on BBP's internal JIRA ticket:

min delay is calculated as:

mindelay_ -= dt;
if ((mindelay_ < 1e-9) || (mindelay_ < dt)) {
   hoc_execerror("usable mindelay is 0", "(or less than dt for fixed step method)");
}

If mindelay_ < dt, the error message still says it's 0. We can be more explicit about this.