Fix JNI NoSuchMethodErrors.

No point calling PrettyMethod if you've just determined that your Method* is
NULL...

Also a few other debugging aids.

Change-Id: I8af520716215800b7ffe4aef784b9c4ec15dcb92
diff --git a/src/object.cc b/src/object.cc
index 0e8e176..f8c9624 100644
--- a/src/object.cc
+++ b/src/object.cc
@@ -433,7 +433,7 @@
     // Do full linkage and set cache value for next call
     result = Runtime::Current()->GetClassLinker()->ResolveType(GetReturnTypeIdx(), this);
   }
-  CHECK(result != NULL);
+  CHECK(result != NULL) << PrettyMethod(this);
   return result;
 }