Test with injected Transport errors

[sourcefrog]

Conserve aspires to be robust against IO errors in archive storage, but this could be better tested.

In particular, cargo-mutants points out that some of the code paths that are meant to handle this in blockdir.rs aren't well tested.

I started using failpoints but perhaps a better approach is to allow configuring some errors into the Transport, since we already have that chokepoint.

For now some mutants are turned off in mutants.toml; they could be re-enabled when this is fixed.

[linear]

CON-85 Test with injected Transport errors