Downgrade non-default builders error to warning

Bug, feature request, or proposal:

Feature request for schematics

What is the expected behavior?

Not having the default builders and using the schematics should throw a warning.

Ideally, there would also be an option to silence it.

What is the current behavior?

Having non-default builders throws an error

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.
StackBlitz starter: https://goo.gl/wwnhMV

npx @nrwl/schematics my-proj --no-interactive
cd my-proj
ng g app app1 --unit-test-runner jest --no-interactive
ng add @angular/material

What is the use-case or motivation for changing an existing behavior?

Users should be able to use their own builders at their own risk.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Whichever versions material enforce the builders (6+?)

Is there anything else we should know?

Original Issue: https://github.com/nrwl/nx/issues/868

[devversion]

I see, reading the config for a builder sounds very reasonable, but I’m afraid that this is out-of-scope for our Material schematics.

Maybe we can work on some public API (maybe there already is one, not too sure yet), and then we can consider using that API in order to check if there is a styles option available.

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