amicommon: set the return value of SysErrorMessage to something default. this is consistent with some other platforms

git-svn-id: trunk@34591 -

rtl/amicommon/sysutils.pp

@@ -764,7 +764,7 @@ end;
 function SysErrorMessage(ErrorCode: Integer): String;
 
 begin
-{  Result:=StrError(ErrorCode);}
+  Result:=Format(SUnknownErrorCode,[ErrorCode]);
 end;
 
 function GetLastOSError: Integer;