fix(sass): remove usage of colors other than primary, improve error

[brandyscarney]

The best way to test this:

1. Create a starter app
2. Remove all colors from the $colors map except for primary
3. Run ionic serve, see the sass errors
4. Change the theme to dark and verify it still errors
5. Update to this branch and link the app to this repo: https://github.com/driftyco/ionic/blob/master/scripts/README.md#developing-against-ionic-locally
6. Repeat the above steps but verify there are no errors.
7. Add a function call to one of the missing colors and check that the error message has improved. Ex: https://github.com/driftyco/ionic-conference-app/blob/master/src/pages/schedule-filter/schedule-filter.scss#L3

fixes #8266

[brandyscarney]

@manucorporat Let me know what you think about this change. The downside is the secondary and danger variables can't be tweaked to change the input highlight, but we have been trying to keep it to where primary is the only required variable.

[manucorporat]

@brandyscarney I have several concerns about the purpose of this PR.
It makes everything more complicated, if I want to change the color of something a have to modify three variable and that's just for one element.

I think it makes harder to customize an ionic app.

I have some ideas, I will comment with you in slack.