Added error handling for the case when Brotli is not imported scrapy#4697

[KishanPipariya]

I added the necessary error generation. Are any further changes required?

Closes: #4697

[wRAR]

I don't think it should be ImportError and I'm also not 100% sure it should be an exception at all (but actually running a callback here is not that useful as it certainly won't get the correctly decompressed response).

[codecov]

Codecov Report

Merging #5480 (54c14b9) into master (8c3e5a2) will decrease coverage by 0.20%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #5480      +/-   ##
==========================================
- Coverage   88.77%   88.57%   -0.21%     
==========================================
  Files         163      163              
  Lines       10666    10668       +2     
  Branches     1818     1819       +1     
==========================================
- Hits         9469     9449      -20     
- Misses        922      942      +20     
- Partials      275      277       +2     

Impacted Files	Coverage Δ
scrapy/downloadermiddlewares/httpcompression.py	90.00% <50.00%> (-2.65%)	⬇️
scrapy/robotstxt.py	75.30% <0.00%> (-22.23%)	⬇️
scrapy/core/downloader/__init__.py	90.97% <0.00%> (-1.51%)	⬇️

[KishanPipariya]

What could be an appopriate response by the program?

[KishanPipariya]

What kind of response should the user receive?

[Gallaecio]

Could you also address the 1st bullet point of #4697?

I imagine we should create a boolean self._can_decode_br in __init__, and then in process_request checking if the Accept-Encoding header exists in the request, and self._can_decode_br is False, and br is one of the encodings in the header. If so, we log a warning.

I am also thinking that, instead of raising an exception if the response is br-encoded, we could remove br from Accept-Encoding in process_response, in addition to logging a warning (that would now also indicate the automatic drop of the unsupported encoding from the header).

[KishanPipariya]

Will try to implement your suggestions
Thanks for the help