Protocol errors did not support async error handlers

[Tronic]

Fix async error handlers which were not being awaited when errors occurred within protocol code (e.g. Payload Too Large).

This appears to be working but I did not review what consequences does it have that server.write_error is now in fact asynchronous.

Issue #1785

[codecov]

Codecov Report

Merging #1790 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1790      +/-   ##
==========================================
+ Coverage   92.08%   92.17%   +0.08%     
==========================================
  Files          23       23              
  Lines        2312     2312              
  Branches      426      426              
==========================================
+ Hits         2129     2131       +2     
+ Misses        141      139       -2     
  Partials       42       42

Impacted Files	Coverage Δ
sanic/handlers.py	97.72% <0%> (+2.27%)	⬆️

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[Tronic]

A potential problem: since protocol does not await for error response being sent, it might be so that another request gets handled in the middle and the responses get mixed up. Could someone familiar with the protocol code have a look at that?

[Tronic]

Closing in favour of #1791 where this is properly fixed.