fix: improve error when context is cancelled during retry backoff

[jooola]

Improve the error message when the context is cancelled while waiting for the retry backoff.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.41%. Comparing base (219a5d1) to head (a7f4ea0).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
hcloud/client_handler_retry.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #814   +/-   ##
=======================================
  Coverage   79.41%   79.41%           
=======================================
  Files          60       60           
  Lines        4566     4566           
=======================================
  Hits         3626     3626           
  Misses        637      637           
  Partials      303      303           

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[jooola]

Should we maybe add details about the last response error we received, giving the reason of the retry?

e.g. rate_limit_exceeded