Give a more descriptive error when the Locust or TaskSet has no tasks.

[cyberw]

Give the user gets a better exception than IndexError: Cannot choose from an empty sequence

Should help with user problems like this one for instance https://stackoverflow.com/questions/60629518/error-while-executing-basic-code-in-locust

[codecov]

Codecov Report

Merging #1287 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1287      +/-   ##
==========================================
+ Coverage   80.12%   80.28%   +0.16%     
==========================================
  Files          23       23              
  Lines        2068     2070       +2     
  Branches      319      320       +1     
==========================================
+ Hits         1657     1662       +5     
+ Misses        333      330       -3     
  Partials       78       78

Impacted Files	Coverage Δ
locust/core.py	91.32% <100%> (+0.07%)	⬆️
locust/rpc/zmqrpc.py	92.1% <0%> (+7.89%)	⬆️

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[cyberw]

Merging this unless someone objects today :) @heyman

[heyman]

Looks good to me!