Display a nicer message when using a Java version incompatible with XWiki

Starting Jetty on port 8080, please wait...
2016-04-12 14:10:01.273:INFO::main: Logging initialized @226ms
java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.eclipse.jetty.start.Main.invokeMain(Main.java:321)
        at org.eclipse.jetty.start.Main.start(Main.java:817)
        at org.eclipse.jetty.start.Main.main(Main.java:112)
Caused by: java.lang.UnsupportedClassVersionError: org/xwiki/tools/jetty/listener/NotifyListener : Unsupported major.minor version 52.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
        at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
        at org.eclipse.jetty.util.Loader.loadClass(Loader.java:86)
        at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.nodeClass(XmlConfiguration.java:364)
        at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.newObj(XmlConfiguration.java:754)
        at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.itemValue(XmlConfiguration.java:1125)
        at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.value(XmlConfiguration.java:1030)
        at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.call(XmlConfiguration.java:721)
        at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.configure(XmlConfiguration.java:417)
        at org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration.configure(XmlConfiguration.java:354)
        at org.eclipse.jetty.xml.XmlConfiguration.configure(XmlConfiguration.java:262)
        at org.eclipse.jetty.xml.XmlConfiguration$1.run(XmlConfiguration.java:1243)
        at java.security.AccessController.doPrivileged(Native Method)
        at org.eclipse.jetty.xml.XmlConfiguration.main(XmlConfiguration.java:1174)
        ... 7 more

Usage: java -jar start.jar [options] [properties] [configs]
       java -jar start.jar --help  # for more information
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