Remove unused variational::base_family class

The base class variational::base_family has no virtual methods and the one function it does define uses no member variables or functions, so can be made standalone outside of the class. Therefore, base_family should be removed.

• remove src/stan/variational/base_family.hpp
• move function write_error_msg_() into its own file and rename without suffix _
• patch families/normal_meanfield.hpp and families/normal_fullrank.hpp:
  • remove references to base_family
  • change calls to write_error_msg_ to include and call new function

If the intent was to somehow pull down some of the functionality from the subclasses, then this issue should be repurposed to do that.