Fix issues with progress == nil causing panics

[smira]

Part of PR #459

This prepares for more methods to be exposed via the API.

[codecov]

Codecov Report

Merging #871 into master will increase coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master    #871      +/-   ##
=========================================
+ Coverage    63.8%   63.8%   +<.01%     
=========================================
  Files          54      54              
  Lines        6655    6661       +6     
=========================================
+ Hits         4246    4250       +4     
- Misses       1885    1886       +1     
- Partials      524     525       +1

Impacted Files	Coverage Δ
http/download.go	77.08% <40%> (-0.94%)	⬇️
deb/remote.go	61.04% <80%> (+0.24%)	⬆️

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