Modifed error message to clarify list_deriv_vars error due to an undefined variable.
Made error messages related to list_deriv_vars more clear.
Merge branch 'mpi' of https://github.com/kenneth-t-moore/OpenMDAO-Framework into mpi
Alterning error messages.
Merge remote-tracking branch 'JustinSGray/dev' into mpi
Added error message specific to VarTrees
Merge branch 'dev' of https://github.com/OpenMDAO/OpenMDAO-Framework into mpi
Reverted assembly.py to original state since deriv var checks reside in Component.check_config.
Merge pull request #1855 from crobinsonut/mpi

Make invalid `list_deriv_vars` errors more specific