Clarify error message when signature in list.txt does not exist.

The previous error didn't really explain the problem from the users point
of view.

Test: modify hiddenapi-unsupported.txt and build
Change-Id: I6f551343db84b20b32a963dccddf10cd08ea2be7

tools/hiddenapi/generate_hiddenapi_lists.py

@@ -188,11 +188,10 @@ class FlagsDict:
     def _check_entries_set(self, keys_subset, source):
         assert isinstance(keys_subset, set)
         assert keys_subset.issubset(self._dict_keyset), (
-            "Error processing: {}\n"
-            "The following entries were unexpected:\n"
+            "Error: {} specifies signatures not present in code:\n"
             "{}"
             "Please visit go/hiddenapi for more information.").format(
-                source, "".join(map(lambda x: "  " + str(x), keys_subset - self._dict_keyset)))
+                source, "".join(map(lambda x: "  " + str(x) + "\n", keys_subset - self._dict_keyset)))
 
     def _check_flags_set(self, flags_subset, source):
         assert isinstance(flags_subset, set)