ref: Remove DownloadError in favor of CacheError

[Swatinem]

Also deduplicates a bunch of downloader code that matches on response statuses.

Fixes #930.

#skip-changelog

[codecov-commenter]

Codecov Report

Base: 65.95% // Head: 66.83% // Increases project coverage by +0.88% 🎉

Coverage data is based on head (87c3826) compared to base (ac4e0a9).
Patch coverage: 59.31% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #956      +/-   ##
==========================================
+ Coverage   65.95%   66.83%   +0.88%     
==========================================
  Files          74       74              
  Lines       11754    11570     -184     
==========================================
- Hits         7752     7733      -19     
+ Misses       4002     3837     -165     

Impacted Files	Coverage Δ
...tes/symbolicator-service/src/services/cficaches.rs	81.94% <ø> (ø)
...mbolicator-service/src/services/download/sentry.rs	22.28% <0.00%> (+3.88%)	⬆️
...es/symbolicator-service/src/services/fetch_file.rs	33.33% <ø> (+16.66%)	⬆️
...s/symbolicator-service/src/services/ppdb_caches.rs	85.89% <ø> (ø)
...symbolicator-service/src/services/symcaches/mod.rs	88.63% <ø> (ø)
crates/symbolicator-service/src/types/objects.rs	94.04% <ø> (+5.03%)	⬆️
crates/symbolicator-service/src/utils/futures.rs	75.00% <ø> (-3.08%)	⬇️
.../symbolicator-service/src/services/download/gcs.rs	50.24% <18.18%> (+8.95%)	⬆️
...rates/symbolicator-service/src/services/derived.rs	85.41% <30.00%> (-14.59%)	⬇️
...s/symbolicator-service/src/services/download/s3.rs	43.95% <38.02%> (+6.63%)	⬆️
... and 13 more

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