Fixed display of last failed command in demula theme

The function that outputs the exit code of the last failed command uses the exit code of any command executed before. That means that currently is using the exit code of the last command executed inside the prompt function (in my case I noticed it when the battery_charge function failed).

By calling the mitsuhikos_lastcommandfailed function in the first line of the prompt function, we make sure the exit code corresponds to the last executed command, and not the commands executed before inside the function.