std.posix.send: should expect ConnectionRefused

Closes: #20219

lib/std/fs/File.zig

@@ -1109,6 +1109,7 @@ pub fn writeFileAll(self: File, in_file: File, args: WriteFileOptions) WriteFile
         error.FileDescriptorNotASocket,
         error.NetworkUnreachable,
         error.NetworkSubsystemFailed,
+        error.ConnectionRefused,
         => return self.writeFileAllUnseekable(in_file, args),
         else => |e| return e,
     };

lib/std/posix.zig

@@ -6088,6 +6088,9 @@ pub const SendError = error{
 
     /// The local network interface used to reach the destination is down.
     NetworkSubsystemFailed,
+
+    /// The destination address is not listening.
+    ConnectionRefused,
 } || UnexpectedError;
 
 pub const SendMsgError = SendError || error{
@@ -6319,7 +6322,6 @@ pub fn send(
         error.AddressNotAvailable => unreachable,
         error.SocketNotConnected => unreachable,
         error.UnreachableAddress => unreachable,
-        error.ConnectionRefused => unreachable,
         else => |e| return e,
     };
 }