message: Add numbers to SEC header to easier lookup
diff: @@ -41,7 +41,7 @@ enum class sec : uint8_t {
  /// Indicates that a request message timed out.
  request_timeout,
  /// Indicates that requested group module does not exist.
  - no_such_group_module,
  + no_such_group_module = 5,
  /// Unpublishing or connecting failed: no actor bound to given port.
  no_actor_published_at_port,
  /// Connecting failed because a remote actor had an unexpected interface.
  @@ -51,7 +51,7 @@ enum class sec : uint8_t {
  /// An actor received an unsupported key for `('sys', 'get', key)` messages.
  unsupported_sys_key,
  /// An actor received an unsupported system message.
  - unsupported_sys_message,
  + unsupported_sys_message = 10,
  /// A remote node disconnected during CAF handshake.
  disconnect_during_handshake,
  /// Tried to forward a message via BASP to an invalid actor handle.
  @@ -61,7 +61,7 @@ enum class sec : uint8_t {
  /// Middleman could not assign a connection handle to a broker.
  failed_to_assign_scribe_from_handle,
  /// Middleman could not assign an acceptor handle to a broker.
  - failed_to_assign_doorman_from_handle,
  + failed_to_assign_doorman_from_handle = 15,
  /// User requested to close port 0 or to close a port not managed by CAF.
  cannot_close_invalid_port,
  /// Middleman could not connect to a remote node.
  @@ -71,7 +71,7 @@ enum class sec : uint8_t {
  /// A C system call in the middleman failed.
  network_syscall_failed,
  /// A function received one or more invalid arguments.
  - invalid_argument,
  + invalid_argument = 20,
  /// A network socket reported an invalid network protocol family.
  invalid_protocol_family,
  /// Middleman could not publish an actor because it was invalid.
  @@ -81,7 +81,7 @@ enum class sec : uint8_t {
  /// Serialization failed because there was not enough data to read.
  end_of_stream,
  /// Serialization failed because no CAF context is available.
  - no_context,
  + no_context = 25,
  /// Serialization failed because CAF misses run-time type information.
  unknown_type,
  /// Serialization of actors failed because no proxy registry is available.
  @@ -91,7 +91,7 @@ enum class sec : uint8_t {
  /// Linking to a remote actor failed because actor no longer exists.
  remote_linking_failed,
  /// Adding an upstream to a stream failed.
  - cannot_add_upstream,
  + cannot_add_upstream = 30,
  /// Adding an upstream to a stream failed because it already exists.
  upstream_already_exists,
  /// Unable to process upstream messages because upstream is invalid.
  @@ -101,7 +101,7 @@ enum class sec : uint8_t {
  /// Adding a downstream to a stream failed because it already exists.
  downstream_already_exists,
  /// Unable to process downstream messages because downstream is invalid.
  - invalid_downstream,
  + invalid_downstream = 35,
  /// Cannot start streaming without next stage.
  no_downstream_stages_defined,
  /// Actor failed to initialize state after receiving a stream handshake.
  