Fix bug in comm based server side validation,

[jdfreder]

which was causing "Widget Javascript not detected" to be
printed to the console despite the Javascript being
installed correctly

cc @SylvainCorlay

[SylvainCorlay]

👍
We could define this loud error outside of the function (and make it a loud warning :) ). For deprecations like #288, users should not have to enable warning manually.

[jdfreder]

For deprecations like #288, users should not have to enable warning manually.

I think the difference between this and that is that this also uses self.log.warn, which prints to the server's terminal window. I do this because I want to make sure that the message is seen somewhere.

[minrk]

@SylvainCorlay IPython should be showing deprecation warnings by default if they are caused directly by user code.