publish: check if storage exists

[neolynx]

Fixes #1477

Description of the Change

Check if storage actually exists in GetPublishedStorage, or return an error (instead of crashing).

Checklist

• unit-test added/updated

[codecov]

Codecov Report

❌ Patch coverage is 52.00000% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.36%. Comparing base (29e643c) to head (dd85493).

Files with missing lines	Patch %	Lines
context/context.go	50.00%	9 Missing ⚠️
deb/publish.go	42.85%	4 Missing and 4 partials ⚠️
cmd/serve.go	0.00%	4 Missing ⚠️
api/repos.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1587      +/-   ##
==========================================
- Coverage   77.43%   77.36%   -0.07%     
==========================================
  Files         165      165              
  Lines       15707    15726      +19     
==========================================
+ Hits        12162    12167       +5     
- Misses       2344     2353       +9     
- Partials     1201     1206       +5     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.