21067: Better handling of missing output/logs on process panel.

21067: Better handling of missing output/logs on process panel.

A few other important changes:

* Similar to the change in #21077, this removes the default error
snackbar popups any time a 400 error happens.  This reduces user
confusion, particularly when "harmless" errors would occur.

* The collection files component will no longer give an "unhandled
rejection" React failure when it can't load the file list (this took
forever to track down.)

* Collections, projects and workflows will now default to a "not
found" panel if they fail to load.  Previously, collections and
workflows would show nothing at all, and projects would show an empty
list.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>