Allow specifying a fallback task

[jjcarstens]

In very specific cases, it might be desirable to fallback to another
task when the target task fails. Most commonly this will be falling
back to the complete task when the upgrade fails due to the firmware
not having a previous firmware.

The fallback will only occur if the failure is withing the tasks functions
so checks before and after the task do not trigger a fallback (i.e. issues
with archive or config file)

This is inspired by nerves-project/nerves#682

[fhunleth]

Thanks!

There's already a task failback code with --task, but it's inflexible. It supports falling back to an arbitrary number of tasks, but they all have to have the same prefix (like to try upgrade.a, upgrade.b, etc. when you just specify --task upgrade). I think that it can be made more flexible and do what you want without adding a new parameter. This will also have the nice property of letting you have n failbacks rather than just one.

The options that I can think of are to specify --task <task1>[:<task2>...] or to specify --task multiple times. Perhaps it doesn't matter. One concern is making it not look like all tasks would be run. We can't use the normal logical or character, ||, to separate tasks since that will be annoying to type in a shell. Perhaps : would be an ok separator.

I'll think more and take a look. This feels like it could end up being a small change.

[jjcarstens]

I started to notice the inflexible fallback, but didn't quite see how we could make it fallback to another task vs just repeating existing tasks instructions.

I like the idea of N fallbacks. I think using the : delimiter is more favorable than multiple --task

[fhunleth]

Let me take a look this week. I'll post back.