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

A command that failed after startup wrote the error twice on stderr:
as an ERROR log record and as the final ERROR: line. The failure
handlers now log the failure at info level, so the record appears only
under --verbose or --debug, next to the records that led up to it.

A failed blob upload is logged at info level for the same reason. A
restore that stops on a file it cannot restore no longer logs that
file's error, which the final line carries; under --skip-errors it is
still logged.

Model: opus-5-5

internal/cli/snapshot.go

@@ -109,7 +109,7 @@ on Linux), then /etc/vaultik/config.yml.`,
 				return v.CreateSnapshot(opts)
 			}, func(err error) {
 				if errors.Is(err, vaultik.ErrPostBackupPruneFailed) {
-					log.Error("Post-backup prune failed", "error", err)
+					log.Info("Post-backup prune failed", "error", err)
 					ReportErrorf("Backup completed, but %v", err)
 
 					return
@@ -118,13 +118,13 @@ on Linux), then /etc/vaultik/config.yml.`,
 				if errors.Is(err, vaultik.ErrFilesUnreadable) ||
 					errors.Is(err, vaultik.ErrSourcePathEmpty) ||
 					errors.Is(err, vaultik.ErrVolumeNotMounted) {
-					log.Error("Backup completed with an error", "error", err)
+					log.Info("Backup completed with an error", "error", err)
 					ReportErrorf("Backup completed, but %v", err)
 
 					return
 				}
 
-				log.Error("Snapshot creation failed", "error", err)
+				log.Info("Snapshot creation failed", "error", err)
 				ReportErrorf("Snapshot creation failed: %v", err)
 			})
 		},
@@ -258,7 +258,7 @@ func newSnapshotVerifyCommand() *cobra.Command {
 					return
 				}
 
-				log.Error("Verification failed", "error", err)
+				log.Info("Verification failed", "error", err)
 				ReportErrorf("Verification failed: %v", err)
 			})
 		},