Delineate subcluster failure reasons by recovery path

[rekmarks]

Requires: #979

When a subcluster fails to start, we want the user to be able to recover. Currently, two reasons present themselves:

1. One or more vats fail to start due to "corrupted" state (meaning actual on-disk corruption or any state that its startup routine fails to handle)
   • Ref: Introduce subcluster / vat upgrades #981
2. One or more vats fail to start because their bundles can't be found
   • Ref: Consider caching vat bundles #978