If validate_hashes() fails, it should leave some breadcrumbs

[leftwo]

In downstairs/src/region.rs, we validate_hashes().
If that validation fails, we should leave behind some breadcrumbs to help diagnose why
there is a validation failure.

                if computed_hash != ctx.hash {
                    error!(self.log, "Failed write hash validation");
                    // TODO: print out the extent and block where this failed!!
                    crucible_bail!(HashMismatch);
                }