Fix missing dependency reporting

[turbolent]

Closes #1703

Description

• API requests may fail, especially due to rate limiting. For contracts with many imports, it's very likely the rate limiting is triggered.

  Retry failing requests a few times, with exponential back-off

• API requests are made during checking, for example to fetch staged contract code. Such requests may fail.

  API request failures are not user errors, but external errors, so treat them as such. Also, do not overwrite such errors as missing dependency errors

@bjartek @austinkline Can you please give this a try and report back if this works better for you?

---

For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 64.93506% with 27 lines in your changes missing coverage. Please review.

Project coverage is 34.40%. Comparing base (b97019a) to head (d165bd5).

Files	Patch %	Lines
internal/migrate/staging_validator.go	43.75%	24 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1706      +/-   ##
==========================================
+ Coverage   34.21%   34.40%   +0.19%     
==========================================
  Files         100      100              
  Lines        6310     6353      +43     
==========================================
+ Hits         2159     2186      +27     
- Misses       3865     3879      +14     
- Partials      286      288       +2     

Flag	Coverage Δ
unittests	34.40% <64.93%> (+0.19%)	⬆️

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