adjust error message when Retry is exhausted

[beliaev-maksim]

related to #574

After debugging I found the difference.
When not a raw response is provided, then error message will be too many error responses. While with raw response it is more explicit and mentions the error code: too many 500 error responses

I added tests that cover it.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (e7834a2) compared to base (27872a4).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #580   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         9    +2     
  Lines         2659      2742   +83     
=========================================
+ Hits          2659      2742   +83     

Impacted Files	Coverage Δ
responses/__init__.py	100.00% <ø> (ø)
responses/tests/test_responses.py	100.00% <100.00%> (ø)
responses/_recorder.py	100.00% <0.00%> (ø)
responses/tests/test_recorder.py	100.00% <0.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.