Add friendlier message about expected limit

[parberge]

Will now print something like this to standard out:

[2020-09-17 09:39:33,411] 7131dd94fc3b/WARNING/locust.main: System open file limit '2048' is below recommended setting '10000'. It's not high enough for load testing, and the OS didn't allow locust to increase it by itself. See https://github.com/locustio/locust/wiki/Installation#increasing-maximum-number-of-open-files-limit for more info.

Since the expected setting isn't documented without this message it's
hard to know what the recommended value is.

[codecov]

Codecov Report

Merging #1566 into master will increase coverage by 0.69%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1566      +/-   ##
==========================================
+ Coverage   81.38%   82.07%   +0.69%     
==========================================
  Files          28       28              
  Lines        2557     2572      +15     
  Branches      392      392              
==========================================
+ Hits         2081     2111      +30     
+ Misses        381      368      -13     
+ Partials       95       93       -2     

Impacted Files	Coverage Δ
locust/main.py	20.08% <0.00%> (-0.18%)	⬇️
locust/clients.py	90.19% <0.00%> (-4.91%)	⬇️
locust/argument_parser.py	77.19% <0.00%> (-0.69%)	⬇️
locust/stats.py	90.08% <0.00%> (+0.06%)	⬆️
locust/user/task.py	96.21% <0.00%> (+0.56%)	⬆️
locust/runners.py	83.86% <0.00%> (+4.43%)	⬆️

---

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[cyberw]

The recommended limit is actually "as high as possible". The reason I chose 10000 is because that is the highest we can automatically increase it to on macOS. Not sure what would be the best way to describe it...

[cyberw]

Overall I prefer a short log entry but maybe some more details in the wiki entry.

[parberge]

@cyberw Ok, so what action should I take? I'm not against any of the suggestions, but I'm not sure what you want me to do with this PR.

[cyberw]

I guess if you just change the word ”recommended” to ”minimum” that is good enough for me to merge.

[cyberw]

Dont forget to format using Black :) (if you can configure your editor to automatically do that, then feel free to add that config to the repo, like it is for VS Code at the moment)

[parberge]

@cyberw I do have black with vscode. So my changes are formatted with black.

However I chose not to include the formatting changes that had nothing to do with my changes to make it clear.
I can include those if you really want

[parberge]

Oh you have that as a step in CI. I see.
I'll include those changes as well then.

[cyberw]

thanks!