Use proper exception type in documentation (bazelbuild#557)

As the outermost exception seen by the user is actually
NoClassDefFoundError the FAQ entry should also mention this type.

org.jacoco.doc/docroot/doc/faq.html

@@ -141,7 +141,9 @@ <h3>Does JaCoCo run on the IBM JRE J9 and WebSphere?</h3>
   <code>-Xshareclasses:none</code>.
 </p>
 
-<h3>Why do I get an error "<code>ClassNotFoundException: org.jacoco.agent[...]Offline</code>"?</h3>
+<h3>Why do I get a <code>NoClassDefFoundError</code> or
+    <code>ClassNotFoundException</code> for class
+    <code>org.jacoco.agent[...]Offline</code>?</h3>
 <p>
   If you use <a href="offline.html">offline instrumentation</a> the instrumented
   classes get a direct dependency on the JaCoCo runtime. Therefore