pm:enable should fail by default if hook_requirements() is not met

[weitzman]

Fixes #4754

[simesy]

Just noting that drush will throw an error, but the caller can bypass the requirement (it's not possible to ignore the requirement in the UI).

I have a module that should never be installed on production (it creates test content) and so I throw an exception in hook_requirements 'install' phase if it's CLI on production.