Mk/bsd.java.mk: Strict JAVA_{VERSION,VENDOR,ERROR} validation

Currently, validation of JAVA_{VERSION,VENDOR,ERROR} does not constitute an
error and make(1) will happily continue, potentially doing the wrong thing.
Adjust the behavior as follows:

• Only validate each var if it's defined
• Remain verbose about what error exists and what's expected
• Fail the build if validation fails

bsd.java.mk exhibited this behavior up until r217132 inadvertently changed
it to be non-fatal.

PR: 220737
Approved by: portmgr (bdrewery)