Warn if spawn rate is too high. Adresses bug #1174 for example.

[cyberw]

No description provided.

[codecov]

Codecov Report

Merging #1211 into master will increase coverage by 0.34%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1211      +/-   ##
==========================================
+ Coverage    78.8%   79.15%   +0.34%     
==========================================
  Files          20       20              
  Lines        1958     1962       +4     
  Branches      308      310       +2     
==========================================
+ Hits         1543     1553      +10     
+ Misses        336      329       -7     
- Partials       79       80       +1

Impacted Files	Coverage Δ
locust/runners.py	79.55% <0%> (+0.2%)	⬆️
locust/core.py	87.71% <0%> (ø)	⬆️
locust/stats.py	85.71% <0%> (+1.36%)	⬆️

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