Handle non-string error values in error responses

[RazerM]

Only set the Space-Track error message from the JSON 'error' value when
it is a string; anything else falls back to showing the raw response
body. A truthy non-string value previously raised TypeError from the
message concatenation instead of the intended HTTPStatusError, so
callers catching HTTPStatusError missed the failure. No such response
has been observed from Space-Track; this is defensive hardening of the
error reporting path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.31%. Comparing base (168de31) to head (18ab5c0).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   96.33%   98.31%   +1.97%     
==========================================
  Files           7        7              
  Lines        1147     1304     +157     
  Branches      129      135       +6     
==========================================
+ Hits         1105     1282     +177     
+ Misses         25       12      -13     
+ Partials       17       10       -7     

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