Do not log warning when one of manifest.json or manifest.webmanifest is retrieved

[dlongley]

Sites generally (and are expected to) publish just one of manifest.json or manifest.webmanifest. The mediator tries to load both -- but always prints a warning for the one that isn't found to the console. This warning should only be printed to the console if both aren't found, otherwise it is almost always a superfluous and confusing warning message.