Problem: :echomsg and :echoerr do not handle List and Dict like :echo does.
(Daniel Hahler)
Solution: Be more tolerant about the expression result type.
461a7fcfce
Add lua functional tests for :echo,:echon,:echomsg,:echoerr
because nvim did not port "test_" functions from Vim
that modify internal state.
Testing :echoerr via try/catch is sufficient.