Error messages

[brandmaier]

I just got this:

julia> model = Sem(
           specification = partable,
               data          = data
       )
ERROR: MethodError: no method matching /(::String, ::Int64)
The function `/` exists, but no method is defined for this combination of argument types.

I realized this happens because I accidentally modeled a variable that was type String. I think we should have a check to catch this and give a proper error message.

[niro4-1]

Consider adding documentation to clarify the cause and resolution for the type mismatch error mentioned in this issue.