Stalled dependency builds cause issues for parents

[guybedford]

When a dependency is taking a very long time to build, this also can cause build errors for parent dependencies since they cannot be built and then can themselves be marked as errored builds, even if the dependency build was just a very slow build and not even an error.

There are a number of things that need to be done here:

• Have a different type of error for pending dependencies as opposed to errored dependencies that can be easily reflushed
• Being able to properly prioritise a dependency build instead of just requeueing the parent build

Fixing the above on the server should be a high priority.