mrc-1259: Better error message when cancelling non-existant task

[richfitz]

This PR prevents an uninformative error while cancelling a non-existant task, replacing it with an informative one. Previously we saw:

obj <- test_rrq()
id <- ids::random_id()
obj$task_cancel(id)
# Error in if (dat$status == TASK_PENDING) { : argument is of length zero

Here, we add a status (TASK_MISSING) if called with a nonexistant task, and the error will come out as

Task abcdef0123456789 is not running (MISSING)"

which is consistent with the case when cancelling a complete task

Task abcdef0123456789 is not running (COMPLETE)"

[codecov]

Codecov Report

Merging #16 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #16   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          17     17           
  Lines        1012   1014    +2     
=====================================
+ Hits         1012   1014    +2

Impacted Files	Coverage Δ
R/rrq_controller.R	100% <100%> (ø)	⬆️

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