Remove 500 codes from HTT_API.md

[andrewazores]

500 Internal Server Error does not need to be documented for each API endpoint, since it's a possibility on any request and may happen for any reason. Specifying potential causes in the API document may mislead readers since this list of causes is almost certainly non-exhaustive, and is subject to change with the implementation - therefore, the 500 should not be counted as part of the published/stable API.