Add option to disable error message on process exit

[eykrehbein]

When an error has been thrown in any of the tasks, this kind of error will be thrown at the end of all tasks:

ListrError: Something went wrong
    at tasks.then (/Users/_/Development/projects/_/node_modules/listr/index.js:117:19)
    at process._tickCallback (internal/process/next_tick.js:68:7)

This pull requests adds an optional option to disable this error message. It can be used like:

new Listr(listrArray, {
    errorOnExit: false // not to be confused with exitOnError
});