[3.1] Make LineTooLong exception more detailed about actual data size (GH-2863)

[asvetlov]

(cherry picked from commit 6a60358)

Co-authored-by: Alexey Popravka popravich@users.noreply.github.com

[codecov-io]

Codecov Report

Merging #2917 into 3.1 will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              3.1    #2917      +/-   ##
==========================================
+ Coverage   97.98%   97.98%   +<.01%     
==========================================
  Files          39       39              
  Lines        7501     7504       +3     
  Branches     1317     1318       +1     
==========================================
+ Hits         7350     7353       +3     
  Misses         48       48              
  Partials      103      103

Impacted Files	Coverage Δ
aiohttp/http_parser.py	98.05% <100%> (+0.01%)	⬆️
aiohttp/http_exceptions.py	100% <100%> (ø)	⬆️

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[new issue]: https://github.com/aio-libs/aiohttp/issues/new