explicitly document exit code 1 on error

cmd/restic/cmd_dump.go

@@ -34,7 +34,8 @@ snapshot.
 EXIT STATUS
 ===========
 
-Exit status is 0 if the command was successful, and non-zero if there was any error.
+Exit status is 0 if the command was successful.
+Exit status is 1 if there was any error.
 `,
 	DisableAutoGenTag: true,
 	RunE: func(cmd *cobra.Command, args []string) error {