if the maven home is incorrect, detect the problem more gracefully.

maven-plugin/src/main/java/hudson/maven/MavenModuleSetBuild.java

@@ -575,7 +575,7 @@ public class MavenModuleSetBuild extends AbstractMavenBuild<MavenModuleSet,Maven
                 if(mvn==null)
                     throw new AbortException("A Maven installation needs to be available for this project to be built.\n"+
                                              "Either your server has no Maven installations defined, or the requested Maven version does not exist.");
-                
+
                 mvn = mvn.forEnvironment(envVars).forNode(Computer.currentComputer().getNode(), listener);
                 
                 MavenInformation mavenInformation = getModuleRoot().act( new MavenVersionCallable( mvn.getHome() ));

maven-plugin/src/main/java/hudson/maven/MavenVersionCallable.java

@@ -24,6 +24,7 @@ package hudson.maven;
  * THE SOFTWARE.
  */
 
+import hudson.AbortException;
 import hudson.remoting.Callable;
 
 import java.io.File;
@@ -53,7 +54,15 @@ public class MavenVersionCallable
     {
         try
         {
-            return MavenEmbedderUtils.getMavenVersion( new File(mavenHome) );
+            File home = new File(mavenHome);
+            if(!home.isDirectory())
+            {
+                if (home.exists())
+                    throw new AbortException("Maven Home "+home+" is not a directory");
+                else
+                    throw new AbortException("Maven Home "+home+" doesn't exist");
+            }
+            return MavenEmbedderUtils.getMavenVersion(home);
         }
         catch ( MavenEmbedderException e )
         {