Name each prefix once in restore, verify and purge errors (closes #327)

A missing blob during restore, a failed shallow verify and an invalid
purge duration each printed their prefix twice. FetchBlob now returns
the storage error as is, since restore and deep verify name the blob
themselves; purge returns the duration parser's error unchanged; and a
failed shallow verify wraps "problems found in storage" instead of
"verification failed", which the command already prints. Deep verify
words a storage error other than not found as a blob it could not
check, and its log line no longer says S3.

Already fixed for #314: the
deep verify size message naming S3 and other errors called missing;
tests now check both.
Not tested: the log line.

Model: opus-5-5

internal/vaultik/verify.go

@@ -680,7 +680,7 @@ func (v *Vaultik) verifyManifestAgainstDatabase(
 func (v *Vaultik) verifyBlobExistenceFromDB(
 	blobs []snapshot.BlobInfo, result *VerifyResult,
 ) error {
-	log.Info("Verifying blob existence in S3", "blob_count", len(blobs))
+	log.Info("Verifying blob existence in storage", "blob_count", len(blobs))
 
 	for i, blob := range blobs {
 		// The hash is read from the snapshot database, which is not
@@ -700,7 +700,7 @@ func (v *Vaultik) verifyBlobExistenceFromDB(
 			result.Missing++
 			result.MissingSize += blob.CompressedSize
 		case err != nil:
-			return fmt.Errorf("checking blob %s in storage: %w",
+			return fmt.Errorf("could not check blob %s in storage: %w",
 				blob.Hash, err)
 		case stat.Size != blob.CompressedSize:
 			log.Warn("Blob has the wrong size in storage",