Consistently return JSON for API responses

I’ve noticed that the Gainsight API returns JSON responses for 500 errors, but for 400, 402, and 404, it returns plain text. 

For example: 500 (Unexpected Error) returns:

{
"message": "Internal Server Error"
}

But a 404 (Not Found) error might return something like:

The requested URL /user/email/abc%xyz.com was not found.

Is there a particular reason why a 404 message couldn’t return:

{
"message": "The requested URL /user/email/abc%xyz.com was not found."
}

In short: it would be way easier to handle API responses if they were consistent in their format and structure.

• Everything should be JSON.
• All errors should have a “message” key
• All successful responses should have a ‘response” key

...or something similar.