io: Always forward IO errors to global handler

The HTTP module raises errors without context. This won't be fixed,
so send them to the global error handler.

python/tests/input_callback.py

@@ -99,6 +99,7 @@ libxml2.registerInputCallback(my_input_cb)
 run_test(desc="Loading entity with custom callback",
         docpath=startURL, catalog=None,
         exp_status="loaded", exp_err=[
+            (  3, 'failed to load "http://example.com/dtds/sample.dtd": Attempt to load network entity\n'),
             ( -1, "Attempt to load network entity http://example.com/dtds/sample.dtd"),
             (  4, "Entity 'sample.entity' not defined\n")
             ])