Improve Errors when Controller Name or Action isn't specfied

These errors occur when, there routes are wrongly defined.

example, the following line would cause a missing :action error

    root "welcomeindex"

Mostly beginners are expected to hit these errors, so lets improve the error message a bit to make their learning experience bit better.
Merge pull request #12740 from gaurish/patch-1

Improve Errors when Controller Name or Action isn't specfied