Improve readability of displayed github api error message

Currently, error messages that are due to errors in github api calls (for example, trying to add an unauthorized assignee to an issue) are displayed in the error snackbar as a raw message with json strings or a very generic error message such as "Validation Failed".

In #933, it is suggested that the error message for validation failed due to assignee could be `Unable to assign the selected user(s) to this issue. Possible reason: a selected user has not joined the GitHub organization yet`.

Let's change the message that is displayed in the error snackbar to make it more readable for the users. Raw error messages should continue to be logged as it may contain additional information that is helpful to us.