feat: Get error status phrase from status code

BREAKING CHANGE: HTTPStatus is new in Python3

ibm_cloud_sdk_core/api_exception.py

@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from typing import Optional
+from http import HTTPStatus
 from requests import Response
 
 
@@ -66,6 +67,8 @@ def _get_error_message(response: Response):
                 error_message = error_json['errorMessage']
             elif response.status_code == 401:
                 error_message = 'Unauthorized: Access is denied due to invalid credentials'
+            else:
+                error_message = HTTPStatus(response.status_code).phrase
             return error_message
         except:
             return response.text or error_message

test/test_api_exception.py

@@ -48,7 +48,7 @@ def test_api_exception():
                   content_type='application/json')
     mock_response = requests.get('https://test-msg.com')
     exception = ApiException(500, http_response=mock_response)
-    assert exception.message == 'Unknown error'
+    assert exception.message == 'Internal Server Error'
 
     responses.add(responses.GET,
                   'https://test-errormessage.com',