UI: Analysis cannot start because of docker image pulls either in progress or failed

[benmandrew]

Context

When the set of docker images increases on a deploy, the deployer must pull the images before any analysis jobs can start running for specific repositories. While this is happening, on the web-UI the analysis jobs appear greyed out and clicking on them results in a 500 server error.

Once the docker pulls complete the analysis jobs start and the correct behaviour ensues. However, if any pull fails behind the scenes, then the analyses remain greyed out indefinitely with no indication that something is permanently wrong.

Expected behaviour

At minimum, clicking on the analysis job should not result in an opaque 500 error. Ideally, while pulls are happening there should be no analysis job visible and there should be some indication in the web-UI that we're waiting on background stuff to process (said in user-friendly terms).

This would help users understand that they just have to wait a little bit, but also help the CI developers to diagnose when a pull has failed, as I have had multiple occasions where I have wasted time trying to debug an issue in the CI that turned out to be failed pulls for whatever reason.

As a non-UI extension, one or two failed image pulls should raise some alarms, but should not prevent the CI from starting analysis jobs for the platforms that succeeded.