Better error messages for unsupported tables

[ire4ever1190]

Tables like this are not supported

type
  Item = object
    name: string
    when not defined(release):
      debugInfo: string

Since there is no way for us to tell if the property should be included in the table or not (Might be possible, but imo would be annoying to implement)

Currently this fails with just a stack trace, error message should be improved to properly tell the user what is wrong. Will implement this once the new testing system from #7 is implemented that allows us to check error messages