Improve installer error message when Java Applet Plug-in selected as JVM on Mac OS X

This is a follow up of JBDS-3014.

When you double click the installer in Finder on OS X, the default java located in /Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home will now be rejected with an error because apparently that is just the JRE, not JDK.

But the error is a bit vague.
Max suggested this:

Should say something like JDK does not work, need to be a full JDK Java 7 or higher or similar if user point to a broken jdk.
and if the folder does not contain anything it should not make it sound that a JDK is actually there

Even better, we could find the location of real JDK and offer that first.