Allow immediate task error

This Pr is a bit like tugging a loose string and has uncovered a few other nasty bits.

The aim is to allow a simple interface to error on task retrieval with task_result and friends and also the bulk functions (like lapply). This is conceptually relatively easy by adding an error = FALSE argument to these functions, and throwing if the user passes error = TRUE and if any of the underling tasks error. This uncovered a few things:

if we throw from an operation that is fetching multiple tasks, we could either throw a container error (which is what I have done) or the first error (which feels like it would be a bit surprising and less useful)
the error thrown when tasks was missing was totally useless - I've tidied that up
bulk submission auto-deleted tasks on completion, which feels weird
I've added an arg single to the internal implementations of tasks_result and tasks_wait so they know if they were called in a scalar context or not (I also renamed task_results to tasks_result` as this was inconsistent)

Things to do later (mrc-4090):

Should it be possible to skip over errors when fetching missing tasks? Should this behave the same way as the error arg?
Should we tidy up the two error types? What does the inheritance path there look like?

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1b2367d) 100.00% compared to head (f4a61c0) 100.00%.

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mrc-ide / rrq

Allow immediate task error #80

Codecov Report