debug hint for missing JNI method

You gotta load the library! Just tripped me up :)

Bug: N/A
Change-Id: Ic6cd8aba53ebf201c214b34ce30a23d4ed858329
Test: N/A
diff --git a/runtime/jni/java_vm_ext.cc b/runtime/jni/java_vm_ext.cc
index f28532b..44ff6eb 100644
--- a/runtime/jni/java_vm_ext.cc
+++ b/runtime/jni/java_vm_ext.cc
@@ -306,6 +306,7 @@
       *detail += "No implementation found for ";
       *detail += m->PrettyMethod();
       *detail += " (tried " + jni_short_name + " and " + jni_long_name + ")";
+      *detail += " - is the library loaded, e.g. System.loadLibrary?";
     }
     return nullptr;
   }