Fix the string representation of ServerDisconnectedError

[JenSte]

What do these changes do?

These changes make sure that the string representation of ServerDisconnectedError is always a valid, human readable string.

Are there changes in behavior for the user?

The user will never be surprised after calling str() on an instance of ServerDisconnectedError.

Related issue number

Fixes #4175.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
• Add a new news fragment into the CHANGES folder

[asvetlov]

I believe providing 'Server disconnected' default value for message could be enough:

if message is None:
    message = "Server disconnected"

__str__ is autogenerated from self.args[0].

[codecov-io]

Codecov Report

Merging #4188 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4188      +/-   ##
==========================================
- Coverage   97.57%   97.56%   -0.02%     
==========================================
  Files          43       43              
  Lines        8825     8825              
  Branches     1381     1381              
==========================================
- Hits         8611     8610       -1     
- Misses         92       93       +1     
  Partials      122      122

Impacted Files	Coverage Δ
aiohttp/client_exceptions.py	100% <100%> (ø)	⬆️
aiohttp/web_fileresponse.py	97.72% <0%> (-0.57%)	⬇️

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[asvetlov]

Thanks!