Verify: report read errors as unreadable, not as corrupted

verifyBlob() caught every exception from loading a blob and counted it as
a failed (corrupted or tampered) blob, so a transient network error during
a deep verify over WebDAV looked like data corruption. Read the range
separately, retry it, and report a read that keeps failing as unreadable
with its error; only a blob that was read but does not decrypt or match
its id counts as failed.

CHANGELOG.md

@@ -14,6 +14,11 @@ instead carried by the app description, the git tag, the release, and this file.
   when the login was more than 30 minutes old: the form did not ask for password confirmation.
 - Settings page: the "Backup and restore" / "Settings" tabs line up with the sections below them.
 - Backups: the runs table scrolls horizontally on narrow screens instead of running off the page.
+- `occ backup:verify --deep`: a blob that could not be read (network or storage error) was reported
+  as "corrupted or tampered". Reads are now retried, and a read that keeps failing is reported
+  separately as unreadable, with the error; only a blob that was read but fails decryption or its
+  hash check counts as corrupted. Found on a Hetzner Storage Box over WebDAV, where the blob
+  turned out to be intact.
 
 ## [0.9.1-beta.2] - 2026-10-07