Option to treat warnings as errors - pure and 100% compliant production build

[ursaj]

Use-case

I'm building production site:

gatsby build

There is a broken link from an inner page to the renamed static image.

Expected

Build failed.

Actual

Warning in output. Build completed.

Scope of the problem

StaticImage is just a single component with such behaviour. There are a lot of other components and plugins that report warnings, while still working. Sometimes incorrectly.

To keep code clean and 100% compliant with all plugins/rules/checks - there should be ZERO warnings in production build output. The point is not in reporter usage (it's out of my and other users control), but how its output is interpreted.

Alternative ways

Indeed I can parse output, e.g. like described on stackoverflow, but to have this option out-of-the-box is much more convenient and simpler for integration.

PR

Default behaviour - unchanged. Those who need such strict mode - would use FAIL_ON_WARNING env variable.

#33404
#33299