Check for server.jar on startup of script

[lubocode]

Checking whether or not the server.jar can be found on script startup should be essential to avoid users setting up something incorrectly and thinking their setup is working, while the script cannot find the server when a users actually tries to connect.

This should be handled with OS independent functions/libraries.

[lubocode]

I just quickly through something together because I needed this in the Docker branch for testing.
Has to be adjusted for usage of the config file and maybe cleaned up a bit.

	// Check if MC server file exists at chosen location
	if mcPath[len(mcPath)-1:] != "/" {
		logger("Path without last \"/\". Adding...")
		mcPath = mcPath + "/"
	}
	mcFilePath := mcPath + mcFile
	logger("mcFilePath: " + mcFilePath)
	if _, err := os.Stat(mcFilePath); err != nil {
		if os.IsNotExist(err) {
			logger("MC server file not found.")
                        os.Exit(1)
		}
	} else {
		logger("MC server file found.")
	}

Oh, and maybe this is Linux specific...
Gotta check the os.Stat() definition.

Also, adding / is definitely linux specific. For Windows one would need to add \

[gekigek99]

added in bc373ba

[lubocode]

Changed to encompass the file and not only the folder in 05c9958