The standard option selected for errors is the hard option (imperative). This means that a message
appears when an error occurs and, if necessary, the activity is aborted. The soft option (non-imperative)
is applied only in the case of a numerical method error, for example due to a lack of
convergence. The activity may be continued, while an error number is added to the parameter
list. The imperative option is set as default, so users need to define explicitly if they
prefer the non-imperative option, by means of a parameter in the parameter list.
Differentiate between errors and warnings.
Always stop if too many, for example 10, errors occur and print a maximum of 10 warnings. The error subroutine entails a STOP only if the number of errors is exceedingly high. A STOP needs to be built into the calling subroutine after fatal errors. In the case of input errors, continue until all errors have been given.