Improve error text message if cannot connect to docker engine

[asvetlov]

No description provided.

[codecov]

Codecov Report

Merging #411 into master will increase coverage by 0.66%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #411      +/-   ##
==========================================
+ Coverage   86.93%   87.60%   +0.66%     
==========================================
  Files          19       19              
  Lines         980      984       +4     
==========================================
+ Hits          852      862      +10     
+ Misses        128      122       -6     

Impacted Files	Coverage Δ
aiodocker/images.py	91.89% <ø> (ø)
aiodocker/__init__.py	100.00% <100.00%> (ø)
aiodocker/docker.py	90.68% <100.00%> (+3.97%)	⬆️

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