Provide more informative SimParam errors

[gregorgorjanc]

Is your feature request related to a problem? Please describe.
Some users are confused with SimParam errors, because error messages are not clear.

Describe the solution you'd like
Can we provide better error messages?

Describe alternatives you've considered
I guess the question is how to test what SimParam setting has already been set and which can be set at will multiple times. Making this clear is likely to enforce/clarify appropriate use case.