How to handle errors when `PendingTask` is part of a group.

[levibostian]

Use case:

• You have a group of 3 PendingTasks added to Wendy.
• Task 1 in that group runs, and fails with an error that the user needs to fix.
• Currently, Wendy deletes tasks when they have an error that the user needs to fix.

What should we do with the remaining 2 tasks in that group?

I believe that Wendy should not delete the task. Currently, that is a good size bug that could cause issues because you're trusting with groups, they must all run in successful order from start to finish.

[levibostian]

Idea:

• When an error occurs in Wendy for a PendingTask in runTask(), do not delete it if it's an error.
• When the task runner runs, check a process when it's time to run it if there is an error recorded for it. If there is, skip running it.
• When a developer calls PendingTasks.addTask() for a PendingTask instance that in the past had an error recorded for it, simply mark the error as resolved and have it try again (also allow via a parameter to not do the default of marking an error as resolved).
• When an error for a PendingTask is resolved, it will be able to run the next time the task runner runs.