Don't throw an error when parsing an invalid js-hypothesis-config JSON
script.

This error thrown by shared.settings#jsonConfigsFrom() was caught in one
of the places where that function is called, but not in other places
where it's called.

Move the error catching and warning logging into the shared function
instead.

This now means, for example, that boot/index.js no longer crashes
(bringing down the entire app) if the host page contains an invalid
js-hypothesis-config.

Unfortunately since jsonConfigsFrom() is called _twice_ on page load to
read the same js-hypothesis-config objects from the host page (it's
called once by boot/index.js and once by annotator/config/config.js) if
there's an invalid js-hypothesis-config a warning about it will be
logged twice.