Return an error type rather than a tuple of string and int

[lpil]

Hello!

Currently the error value for the command function is very unrefined:

• The fields are unnamed, so it's unclear what they are without experimentation.
• The error design states there is a status code, but one may not exist if the program was never run.
• The error is represented as a string, so the user has to parse this string in order to determine what the error was.

I think it'd be a lot nicer to use if there was an error type. It could have variants for the different kinds of reasons why the process failed to run.

Thanks, Louis

[tynanbe]

Thanks, I'll look into it when time allows.