Fix exception causes in client.py

[cool-RR]

I recently went over Matplotlib, Pandas and NumPy, fixing a small mistake in the way that Python 3's exception chaining is used. If you're interested, I can do it here too. I've done it on just one file right now.

The mistake is this: In some parts of the code, an exception is being caught and replaced with a more user-friendly error. In these cases the syntax raise new_error from old_error needs to be used.

Python 3's exception chaining means it shows not only the traceback of the current exception, but that of the original exception (and possibly more.) This is regardless of raise from. The usage of raise from tells Python to put a more accurate message between the tracebacks. Instead of this:

During handling of the above exception, another exception occurred:

You'll get this:

The above exception was the direct cause of the following exception:

The first is inaccurate, because it signifies a bug in the exception-handling code itself, which is a separate situation than wrapping an exception.

Let me know what you think!

[codecov-commenter]

Codecov Report

Merging #4815 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4815   +/-   ##
=======================================
  Coverage   97.60%   97.60%           
=======================================
  Files          43       43           
  Lines        8932     8932           
  Branches     1406     1406           
=======================================
  Hits         8718     8718           
  Misses         95       95           
  Partials      119      119           

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[cool-RR]

If I understand correctly, the test failure here is not related to my change. (I see it's happening on other PRs.)

[codecov-io]

Codecov Report

Merging #4815 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4815      +/-   ##
==========================================
- Coverage   97.66%   97.65%   -0.02%     
==========================================
  Files          43       43              
  Lines        8983     8983              
  Branches     1412     1412              
==========================================
- Hits         8773     8772       -1     
- Misses         97       98       +1     
  Partials      113      113              

Flag	Coverage Δ
#unit	97.65% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	97.84% <0.00%> (-0.54%)	⬇️

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[asvetlov]

Agree, you are right.
The proposal makes value and produces better understandable exception info.
I'll merge this PR; please feel free to apply such improvements to all codebase.

[aio-libs-github-bot]

💚 Backport successful

The PR was backported to the following branches:

• ✅ 3.7