Notify about transient errors when cloning forks from GitHub

[fthomas]

Closes #472.

If cloning forks from GitHub fails, we adapt the error to notify
about the possibility that cloning failed because the fork is not
yet ready.

I think #472 is not severe enough to warrant a more elaborate fix.

[codecov]

Codecov Report

Merging #2285 (320bde1) into master (f601a2b) will decrease coverage by 0.09%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #2285      +/-   ##
==========================================
- Coverage   77.93%   77.84%   -0.10%     
==========================================
  Files         135      135              
  Lines        2293     2297       +4     
  Branches       58       60       +2     
==========================================
+ Hits         1787     1788       +1     
- Misses        506      509       +3     

Impacted Files	Coverage Δ
...n/scala/org/scalasteward/core/vcs/VCSRepoAlg.scala	76.47% <60.00%> (-15.84%)	⬇️

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