strings: add logging for untranslated source messages

This should probably be changed to be closer to the goal of being
like gettext but we can at least tell developers when we clobber the
string
User-visible warning when tile sources fail to load

This is currently using window.alert in lieu of something more sophisticated but it works for indicating 404s or invalid file
contents
Viewer: add a basic error message display system

This might be something we want to make configurable for users who want
to completely control how errors are displayed.

This also adds the first use of OpenSeadragon.makeCenteredNode currently
so we might want to clean up that code as well.