GitHub API returns JSON object instead of array when encountering errors
(e.g., rate limit exceeded). This causes JSON unmarshal error:
'cannot unmarshal object into Go value of type []service.Release'
Add HTTP status code check to handle error responses gracefully and
return user-friendly error messages instead of JSON parsing errors.
Fixes issue where getXrayVersion fails with unmarshal error when
GitHub API rate limit is exceeded.
c881d1015a