Failure to parse exit command args should return 2 exit code

[dsherret]

Just found this comment in Dax (written by someone else, not me):

https://github.com/dsherret/dax/blob/c885ba58ba52035a6cf99629dbcac90209a94604/src/commands/exit.ts#L13

This line should be changed to 2 it seems:

https://github.com/denoland/deno_task_shell/blob/26ef38332e0156136bd7bd48571753f715c18204/src/shell/commands/exit.rs#L26