Provide a better error message when the mojo descriptor is null.

diff --git a/maven-plugin-testing-harness/src/main/java/org/apache/maven/plugin/testing/AbstractMojoTestCase.java b/maven-plugin-testing-harness/src/main/java/org/apache/maven/plugin/testing/AbstractMojoTestCase.java
index 08cd9aa..a1c7a66 100644
--- a/maven-plugin-testing-harness/src/main/java/org/apache/maven/plugin/testing/AbstractMojoTestCase.java
+++ b/maven-plugin-testing-harness/src/main/java/org/apache/maven/plugin/testing/AbstractMojoTestCase.java

@@ -409,7 +409,7 @@
     protected MojoExecution newMojoExecution( String goal )
     {
         MojoDescriptor mojoDescriptor = mojoDescriptors.get( goal );
-        assertNotNull( mojoDescriptor );
+        assertNotNull(String.format("The MojoDescriptor for the goal %s cannot be null.", goal),  mojoDescriptor );
         MojoExecution execution = new MojoExecution( mojoDescriptor );
         finalizeMojoConfiguration( execution );
         return execution;