Sass helpers: Change all warnings to errors

[benjamincharity]

Our Sass helper functions/mixins should alert the user if used incorrectly. Angular CLI seems to be suppressing @warn messages from our helpers so we need to switch them to @error.

IMO it's not the best workflow to stop compilation for style errors. But since a warning doesn't make it through to the consumer this is the only way to avoid very difficult to track down bugs.

Reproduction

1. Pass an incorrect value to a Sass helper (e.g. z-index: z(foo);)
2. See no warning in the cli

Which versions of this library, Angular, TypeScript, & browsers are affected?

• UI Library: all versions
• Angular: v7
• TypeScript: 3.1.6

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• Change all instances of @warn in our Sass files to @error

[shani-terminus]

I noticed in my local after a bad input was corrected, I would sometimes get an error: Cannot read property 'toString' of undefined. This went away when I stopped the build:ts:watch and ran it again.