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/shallow_verify_test.go

@@ -100,7 +100,8 @@ func TestShallowVerifyDetectsWrongBlobSize(t *testing.T) {
 	// The cli layer prints this error after its own "Verification failed:".
 	err = newVerifier(io.Discard).VerifySnapshotWithOptions(
 		snapshotID, &vaultik.VerifyOptions{})
-	require.EqualError(t, err, "1 blobs have the wrong size")
+	require.EqualError(t, err,
+		"problems found in storage: 1 blobs have the wrong size")
 }
 
 // TestShallowVerifyDetectsMissingDatabase backs up a real snapshot,

internal/vaultik/snapshot.go

@@ -23,6 +23,7 @@ var (
 	errSnapshotNotInConfig  = errors.New("snapshot not found in config")
 	errNoSnapshotsInConfig  = errors.New("no snapshots configured")
 	errSnapshotVerifyFailed = errors.New("verification failed")
+	errProblemsInStorage    = errors.New("problems found in storage")
 	errRemoveAllNeedsForce  = errors.New("--all requires --force")
 	errSnapshotNotFound     = errors.New("snapshot not found")
 	errSnapshotRemoveFailed = errors.New("snapshot(s) could not be removed")
@@ -907,13 +908,6 @@ func (v *Vaultik) verifyManifestBlobs(
 	return verified, missing, mismatched, missingSize, unchecked, nil
 }
 
-// shallowVerifyError is the error a failed shallow verify returns. Its
-// text is only what failed, because the cli layer prints it after its own
-// "Verification failed:".
-type shallowVerifyError string
-
-func (e shallowVerifyError) Error() string { return string(e) }
-
 // formatVerifyResult outputs the final verification results as JSON or
 // human-readable text.
 func (v *Vaultik) formatVerifyResult(
@@ -948,8 +942,10 @@ func (v *Vaultik) formatVerifyResult(
 		v.printVerifySummary(result, failure)
 	}
 
+	// Not errSnapshotVerifyFailed: the cli layer prints this after its own
+	// "Verification failed:".
 	if failure != "" {
-		return shallowVerifyError(failure)
+		return fmt.Errorf("%w: %s", errProblemsInStorage, failure)
 	}
 
 	return nil