Backport of [ui] Jobs list should handle 502s and 504s gracefully into release/1.8.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #23427 to be assessed for backporting due to the inclusion of the label backport/1.8.x.

The below text is copied from the body of the original PR.

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Previously, our jobs index page would handle handle all errors when fetching jobs by resetting state and trying again (with 5-second backoff).

This PR makes it so that proxy-timeout specific errors instead throw a notification, but keep the cached jobs shown on the page.

The user is then presented with two options to move forward:

• pause fetch attempts (for this session or until they click "Manually fetch jobs")
• Immediately try again (in case they extended their proxy settings in the background and want to immediately cancel pending requests and try again, or restart previously-paused attempts)

Resolves #23228

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Overview of commits

• 54aafa5

[github-actions]

Ember Test Audit comparison

	release/1.8.x	c54235a	change
passes	1563	1564	+1
failures	2	2	0
flaky	0	0	0
duration	000ms	000ms	-000ms

[github-actions]

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