[build] convert external dependencies (missing dependencies) into Rollup errors instead of warnings.

[trusktr]

Rollup by default treats missing dependencies as "external" and will only output a warning to the console instead of an error (working as intended).

We can make it throw an error for our use case.

See: rollup/rollup#1906 (comment)

[mohammedsahl]

I'd like to work on this. How would you suggest going about with testing? Can you tell me of a way to reproduce the missing dependencies warning. @trusktr