Raise consistently on avm_unix uart:open/1,2 failure

The configure-failure branch returned {error, _} while the other two
failure paths raise; the open/1,2 spec only allows {pid(), fd()}.

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

Author: pguyot

libs/avm_unix/src/uart.erl

@@ -91,7 +91,7 @@ open(Opts) ->
                     {Pid, Fd};
                 {error, _} = CfgErr ->
                     atomvm:posix_close(Fd),
-                    CfgErr
+                    error(CfgErr)
             end;
         {error, Reason} ->
             error(Reason)