gazelle: Fail when errors occur

[stevebarrau]

Instead of introducing a fail-fast, leverage the lifecycle to fail the extension when errors that would produce invalid build files occur.

[illicitonion]

LGTM! Can you add an integration test or two showing this working?

java/gazelle/testdata/lib_with_invalid_label_in_runtime_deps has an example of a test where we run gazelle and expect it to fail with specific output.

[stevebarrau]

Added 2 tests:

• one where there is a collision
• one where there is a collision and the developer added the required resolve directive

[illicitonion]

Thanks!