exit with code (1) on build warnings

[pi0]

Build warnings (specially the ones related to externals) can cause the deployment to be failed (example: nitrojs/nitro-deploys#83) and this goes unnoticed during whole CI/CD and deploy pipeline.

I think it would be safer to fail CLI by code (1) by default with an explicit flag to opt-out. (this will be available for nitro CLI, framework integrations such as nuxi need to do it themselves and we need to somehow expose warnings information to them /cc @danielroe)