Parse API Error messages with int error codes

[renaudhartert-db]

Changes

Parsing error messages from the /api/2.0/preview/scim/v2/Me API causes parsing errors because the API returns the HTTP status (i.e. an int) as error code instead of a string. For example:

{
   "error_code": 403,
   "message": "Invalid access token."
}

This PR solves the problem by parsing the error_code field as a string no matter its type.

Note that the implementation is overkill because it also converts any literal to a string (e.g. true becomes "true"). We could guarantee that the behavior is limited to converting int to string by adding a custom Unmarshaler but this didn't seem worth the additional code.

This PR also:

• Refactors the code by removing the APIErroBody struct. The rationale is that the struct exists solely for parsing and is actually not used outside of the parseErrorFromResponse function. Turning it into an anonymous struct within parseErrorFromResponse makes the intent crystal clear.
• Slightly restructures the test cases to better match go's naming conventions and separate the "three tests parts" (init, test, assert).

Tests

Added a unit test to verify that (i) an int error code are parsed as string, and (ii) that this string is properly ignored.

• make test passing
• make fmt applied
• relevant integration tests applied