java: Note that JVM >= 1.6 is now required.
(cherry picked from commit 87d1f7714f8e1b0f595bad3b7932592f0f85d54b)
java: Print a better error message when JVM version is too old.

This fixes commit 87d1f7714f8e1b0f595bad3b7932592f0f85d54b.
(cherry picked from commit 5c6895bda1053b4cbbcc8c72fe30d8fd17d403d6)