* ui: Gracefully recover from non-existent DC errors
This PR fixes what happens in the UI if you try to navigate to a non-existing DC.
When we received a 500 error from an API response due to a non-existent DC, previously we would show a 404 error, which is what we were trying to convey. But in the spirit of the UI being a 'thin client', its probably best to just show the 500 error from the API response, which may help folks to debug any issues better.
* Automatically set the CONSUL_DATACENTER_LOCAL env var for testing
