Issue #332: Hide traceback from UnprocessableEntity error (#336)

* hide UnprocessableEntityError traceback from user

* extract message from HTTPError

* remove suggestion to open issue when 422 is responsed

git_machete/exceptions.py

@@ -14,14 +14,14 @@ def __init__(self) -> None:
         pass
 
 
-class UnprocessableEntityHTTPError(Exception):
+class UnprocessableEntityHTTPError(MacheteException):
     """This exception is raised when Github API returns HTTP status code 422 - Unprocessable Entity.
     Such a situation occurs when trying to do something not allowed by GitHub,
     e.g. assigning someone from outside organization as a reviewer
     or creating a pull request for a branch that already has a PR.
     """
     def __init__(self, msg: str) -> None:
-        self.msg: str = msg + "\nPlease open an issue regarding this topic under link: https://github.com/VirtusLab/git-machete/issues/new"
+        self.msg: str = 'UnprocessableEntityHTTPError: ' + msg
 
     def __str__(self) -> str:
         return str(self.msg)

git_machete/github.py

@@ -147,7 +147,9 @@ def __fire_github_api_request(method: str, path: str, token: Optional[str], requ
             return parsed_response_body
     except HTTPError as err:
         if err.code == http.HTTPStatus.UNPROCESSABLE_ENTITY:
-            raise UnprocessableEntityHTTPError(str(err.reason))
+            error_response = json.loads(err.read().decode())
+            error_reason: str = error_response['errors'][0]['message']
+            raise UnprocessableEntityHTTPError(error_reason)
         elif err.code in (http.HTTPStatus.UNAUTHORIZED, http.HTTPStatus.FORBIDDEN):
             first_line = f'GitHub API returned {err.code} HTTP status with error message: `{err.reason}`\n'
             if token: