Print a failed command's error once on stderr (closes #433)

A failed command printed its error twice on stderr: as an ERROR log record and as the final ERROR: line the README documents. The command handlers, a restore failure that ends the command and a failed blob upload now log that error at info level or not at all, so without --verbose or --debug only the final line appears.

Model: opus-5-5

internal/cli/entry_snapshot_create_prune_test.go

@@ -61,8 +61,8 @@ func TestEntrySnapshotCreateRejectsKeepNewerThanBeforeBackup(t *testing.T) {
 
 // TestEntrySnapshotCreateReportsPostBackupPruneFailure makes the
 // post-backup prune fail after a good backup, by planting a manifest
-// that cannot be decoded, and checks that the failure is reported as the
-// prune's and not as a failed snapshot creation.
+// that cannot be decoded, and checks that the failure is reported once,
+// as the prune's and not as a failed snapshot creation.
 //
 //nolint:paralleltest // replaces os.Args, os.Stdout, os.Stderr and the xdg globals
 func TestEntrySnapshotCreateReportsPostBackupPruneFailure(t *testing.T) {
@@ -83,6 +83,9 @@ func TestEntrySnapshotCreateReportsPostBackupPruneFailure(t *testing.T) {
 	assert.Contains(t, stderr, "Backup completed, but post-backup prune failed")
 	assert.NotContains(t, stderr, "Snapshot creation failed")
 
+	_, repeats := failureOutput(t, stderr)
+	assert.Zero(t, repeats, "no log record may repeat the error:\n%s", stderr)
+
 	snapshots, err := os.ReadDir(filepath.Join(storeDir, "metadata"))
 	require.NoError(t, err)
 	assert.Len(t, snapshots, 2,
@@ -93,7 +96,8 @@ func TestEntrySnapshotCreateReportsPostBackupPruneFailure(t *testing.T) {
 // empties the source directory, as a volume that is not mounted leaves
 // its mount point, and runs `snapshot create --prune`. The command exits
 // non-zero and reports the backup as completed, not as a failed snapshot
-// creation. See https://git.eeqj.de/sneak/vaultik/issues/388.
+// creation. The error is printed once. See
+// https://git.eeqj.de/sneak/vaultik/issues/388.
 //
 //nolint:paralleltest // replaces os.Args, os.Stdout, os.Stderr and the xdg globals
 func TestEntrySnapshotCreatePruneExitsNonZeroWhenSourceIsEmpty(t *testing.T) {
@@ -114,6 +118,9 @@ func TestEntrySnapshotCreatePruneExitsNonZeroWhenSourceIsEmpty(t *testing.T) {
 		"Backup completed, but 1 source path(s) were empty")
 	assert.NotContains(t, stderr, "Snapshot creation failed")
 
+	_, repeats := failureOutput(t, stderr)
+	assert.Zero(t, repeats, "no log record may repeat the error:\n%s", stderr)
+
 	snapshots, err := os.ReadDir(filepath.Join(storeDir, "metadata"))
 	require.NoError(t, err)
 	assert.Len(t, snapshots, 2, "the older snapshot is kept")
@@ -124,7 +131,8 @@ func TestEntrySnapshotCreatePruneExitsNonZeroWhenSourceIsEmpty(t *testing.T) {
 // directory as a mount point, and runs `snapshot create --prune`, which
 // does not find it as one. The command exits non-zero, reports the
 // backup as completed, not as a failed snapshot creation, and keeps the
-// older snapshot. See https://git.eeqj.de/sneak/vaultik/issues/395.
+// older snapshot. The error is printed once. See
+// https://git.eeqj.de/sneak/vaultik/issues/395.
 //
 //nolint:paralleltest // replaces os.Args, os.Stdout, os.Stderr and the xdg globals
 func TestEntrySnapshotCreatePruneExitsNonZeroWhenMountPointIsMissing(
@@ -159,6 +167,9 @@ func TestEntrySnapshotCreatePruneExitsNonZeroWhenMountPointIsMissing(
 		"an earlier snapshot recorded were not found")
 	assert.NotContains(t, stderr, "Snapshot creation failed")
 
+	_, repeats := failureOutput(t, stderr)
+	assert.Zero(t, repeats, "no log record may repeat the error:\n%s", stderr)
+
 	snapshots, err := os.ReadDir(filepath.Join(storeDir, "metadata"))
 	require.NoError(t, err)
 	assert.Len(t, snapshots, 2, "the older snapshot is kept")