v3-beta7: Better error messages when Cordova is not installed

[janpio]

What version of the CLI are you using? Output of ionic --version

3.0.0-beta7

Short description of the problem:

Calling ionic cordova platform outputs a non-descript error when Cordova is not installed:

> ionic cordova platform
[ERROR] Command not found: cordova

What behavior are you expecting?

Calling a cordova sub-command on a project when Cordova is not installed should output this error message but also point the user in the right direction to install Cordova on his machine.

[imhoffd]

Hi! In the next beta release you should see this:

Along with a super helpful, huge error message during preinstall of the plugin:

Thank you! 🎉