* Ignore read errors based on policy settings
Added an error handling policy section. Can independently control error handling for directory and file read errors, toggle-able from the `policy set` command to either "true", "false", or "inherit". If any read error is hit, the error handling will check the effective policy on whether to ignore it or not. Currently there is no differentiation between read error types, though in the future we may want to add the `errors.Is(err, os.ErrPermission)` conditional.
Fix was implemented such that the policy ignores read errors ONLY on child entries of the source. So a snapshot will still fail if the source root directory itself can't be read, but you can ignore the error if a file or a subdirectory in the snapshot source root can't be read. I did this to address some otherwise strange behavior where you would successfully snapshot (because you ignored the error), but couldn't restore that snapshot because nothing really happened during the operation.
