Send back correct error message when action is private

[michielmulders]

Expected behavior

Return proper error message when the action is not public.
--> Referring to discussion leading up to this issue.

Expected error message: The action ACTION_NAME on module MODULE_NAME is not public.

Actual behavior

Not clear from error what has gone wrong.

Steps to reproduce

Define an action as private and call the action.
channel.invoke('chain:mytestaction', ...)

Which version(s) does this affect? (Environment, OS, etc...)

2.*

[pablitovicente]

Maybe we should discuss a little deeper about channels check for other issues we need to check/guard against #3539