Fix error message to mean something useful. If I have to look at the code to resolve a problem, the message + doco is insufficient.

git-svn-id: https://svn.apache.org/repos/asf/maven/maven-1/plugins/trunk@113365 13f79535-47bb-0310-9956-ffa450edef68

uberjar/plugin.jelly

@@ -99,7 +99,7 @@
     <j:if test="${empty(ubermain)}">
       <fail message="">
 
-The ubermain property has not been set.  This value should
+The maven.uberjar.main property has not been set.  This value should
 be set to the value of the class that contains the main method
 to be invoked.