jdtls compiler defaults and Maven's generated project settings classify access restrictions as errors, so on JDK 8 the launch check reported compile errors for code that javac accepts.
- Configure forbidden and discouraged references as warnings through cached preferences and the jdtls settings API, for metadata that jdtls keeps in its own workspace. An existing project `.settings` directory is never touched.
- Real type errors are still reported before launch.
Fixes#536