Modernising old libraries - error handling

[FortranFan]

@Arjen

I tend to prefer returning a NaN, because then it will be immediately clear something is wrong. A value like -999 does have appeal, as it can be exactly represented, but it can also easily be ignored.

I would never use a library that returns NaNs.

Considering the current Fortran standard with rudimentary to no support for this aspect, KISS will be preferable for FOSS libraries in Fortran:

1. FUNCTION subprograms: employ ERROR STOP for the exceptions that, hopefully, are as such!
2. SUBROUTINEs: include STAT and ERRMSG arguments and follow intrinsic procedures: a) STAT is INTENT(OUT) and zero value means SUCCESS and b) ERRMSG is INTENT(INOUT)