Viewers: 404 handling in MPR and other modes

[pieper]

Bug Report

Describe the Bug

It's currently possible for the app to hang or present a black screen when a request gives 404 or otherwise fails.

What steps can we follow to reproduce the bug?

MPR mode makes many frame requests and the server may not be able to handle them.

I was able to get this behavior by opening several tabs with different studies and entering 2D MPR mode on each one. Eventually the IDC throttle proxy stopped responding and I got a black screen on one of the tabs.

I also went into MPR mode on Kheops with a CT and the view hung at 4% with the console full of 404 messages.

Suggested behavior

It would be best to report back to the user that there is trouble communicating with the server, perhaps suggesting that they reload and/or try again later.

[pieper]

Related to #1957, which is about avoiding too many requests, but this one is about graceful handling of failures that might happen anyway.

[pieper]

@JamesAPetts we can discuss on Thursday with @fedorov. Probably this means adding some UI to report errors from the request pool.

[JamesAPetts]

Soungs good, yeah #1957 should help somewhat, but yeah a clean path for error handling without breaking the app in MPR would be nice. Look forward to discussing.