Fixed error message for using OpenJDK.

Output a meaningful error message, when OpenJDK is detected
instead of giving the "wrong java version" output.
That output confused several users (from experiences in IRC)
since it gives you an output like "You are using java
version 1.6.0, right version would be Java SE 1.6".

Change-Id: I31ceeb03f1f98524b2b7de9b8be45231985fdb9e
Signed-off-by: Tim Roes <tim.roes88@googlemail.com>