Unify code and minor fix: Unified folder checks
- catkin_make and catkin use same logic and function
- fix slight inconsistency in catkin_make, if
  source_space == None, assumed to be 'base_path' in first location,
  base_path/src in second location
- source_space var not unnecessarily set joined to base_path
- Introduced ValidationError for pretty Error in c_m_i