[PATCH] Improve error message when trying to replay a directory

[elexis]

If you accidentally give a directory name instead of the path to the commands.txt file when attempting to go into visual- or non-visual replaymode, then you will get the following error:

terminate called after throwing an instance of 'std::ios_base::failure'
  what():  basic_filebuf::underflow error reading the file

As we already have a nice error in case the file doesn't exist, we should check that case too:

static void RunGameOrAtlas(int argc, const char* argv[])
...
		if (!FileExists(OsPath(replayFile)))
		{
			debug_printf("ERROR: The requested replay file '%s' does not exist!\n", replayFile.c_str());
			return;
		}

[elexis]

In 17501:

Graceful exit in case of trying to replay a directory. Fixes #3427.
Also remove unneeded variable ran_atlas.