small refactor for retry downloader

[a1012112796]

add a new function to do retry logic.

[codecov-commenter]

Codecov Report

Merging #16137 (03597ca) into main (0bf8d34) will increase coverage by 0.23%.
The diff coverage is 48.77%.

@@            Coverage Diff             @@
##             main   #16137      +/-   ##
==========================================
+ Coverage   44.02%   44.25%   +0.23%     
==========================================
  Files         681      694      +13     
  Lines       82336    82299      -37     
==========================================
+ Hits        36245    36422     +177     
+ Misses      40185    39991     -194     
+ Partials     5906     5886      -20     

Impacted Files	Coverage Δ
cmd/serv.go	2.58% <0.00%> (-0.04%)	⬇️
cmd/web.go	0.00% <0.00%> (ø)
models/avatar.go	32.30% <ø> (ø)
models/consistency.go	39.57% <0.00%> (-1.01%)	⬇️
models/error.go	39.53% <0.00%> (-0.32%)	⬇️
models/repo.go	53.14% <0.00%> (-1.11%)	⬇️
modules/auth/ldap/ldap.go	46.81% <ø> (ø)
modules/avatar/avatar.go	47.72% <ø> (ø)
modules/base/tool.go	93.47% <ø> (-0.79%)	⬇️
modules/git/commit.go	60.16% <ø> (+3.69%)	⬆️
... and 193 more

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