Include errors in current task and synchronise on begin_task

[callumforrester]

Changes:

• Replace the is_error field on TrackableTask with errors, a list of strings
• Include all errors generated while a task is active in its errors
• Synchronise begin_task so it blocks until the runner thread has set the active task, allows:

worker.begin_task(my_id)
active = worker.get_active_task()
assert active.task_id == my_id

[codecov]

Codecov Report

Merging #234 (194a78d) into main (d31287f) will decrease coverage by 0.23%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
- Coverage   89.90%   89.68%   -0.23%     
==========================================
  Files          41       41              
  Lines        1248     1260      +12     
==========================================
+ Hits         1122     1130       +8     
- Misses        126      130       +4     

Impacted Files	Coverage Δ
src/blueapi/worker/reworker.py	94.76% <91.66%> (-1.73%)	⬇️
src/blueapi/worker/worker.py	100.00% <100.00%> (ø)

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