[BUG] Consider making 'You aren't using a compiler supported by lombok' diagnostic an error (instead of a warning)

[cushon]

Describe the bug

When lombok runs in a compiler it doesn't support, it logs the following WARNING level diagnostic:

https://github.com/rzwitserloot/lombok/blob/04f17f4dcfbb0e7881d605708467498e3ec56521/src/core/lombok/core/AnnotationProcessor.java#L222

Because the diagnostic is only an error, the compilation can still succeed. In practice, the build will usually failure later when someone tries to reference the methods that lombok would have generated. (bazelbuild/bazel#12837 is an example where the downstream error was seen, but not the warning about the underlying problem.)

I wonder if it that diagnostic should be an error? In cases where the build is going to fail anyways, I think it would be more helpful to fail early with the diagnostic about the unsupported compiler. (Perhaps there are situations where users are relying on this diagnostic being just a warning?)

To Reproduce

The repro is just to run lombok on a compiler it doesn't support, e.g. that isn't javac or ecj.

Version info (please complete the following information):

• lombok 1.18.16
• openjdk version 11.0.6+10-LTS
• annotation processing implementation: https://github.com/google/turbine