[uiAppTemplate] display errors on screen when resources fail to load

[spalger]

This simply displays errors on the loading screen when a resource fails to load. These types of errors usually mean that there is an error in the code, and generally require that we keep our eyes on the error console while the page loads. This is a bit more obvious I think...

[kimjoar]

I like this!

Btw, is this always because the build failed and you therefore can't load the file? If so, could we just show one error message, but with a more "descriptive" error msg? Might make it easier to understand what has happened for those building Kibana plugins (otherwise, "why did it fail to load!?"). Maybe something along the lines of:

The build process wasn't able to build all the files. To debug the problem see the Kibana terminal output.

[spalger]

Build failures are by far the most common reason files would fail to load, but there are other reasons. I suppose it wouldn't hurt to at least expand the message to say something like check "check the kibana server".

[kimjoar]

LGTM 👍