Improve formatting of UnexpectedResponse.getMessage

[fthomas]

This lists each header of the response on its own line instead of
having them all on one line.

[codecov]

Codecov Report

Merging #2356 (30b2ad5) into master (cc04fcf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2356   +/-   ##
=======================================
  Coverage   80.16%   80.16%           
=======================================
  Files         138      138           
  Lines        2374     2375    +1     
  Branches       55       57    +2     
=======================================
+ Hits         1903     1904    +1     
  Misses        471      471           

Impacted Files	Coverage Δ
...la/org/scalasteward/core/util/HttpJsonClient.scala	51.72% <100.00%> (+1.72%)	⬆️

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