Test that cache errors are non-fatal (#685)
The test creates a cache from multiple sources and injects faults (once
using invalid data and once by making the files unreadable on the fs
level), then resolves again.
I didn't test git because it has its own locking and correctness logic.
The main drawback is that this test is slow (2.5s for me), we could
`#[ignore]` it.
