Web3signer keymanager API doesn't send JSON with a 500 code

[yorickdowne]

This is an extremely low priority bug, as it can be worked around on the calling side.

Web3signer will, upon encountering an internal server error on the keymanager API, send back code 500 and the plaintext message Internal Server Error.

It should be sending a JSON:

{
  "message": "Internal Server Error"
}

This can be reproduced by using the v26.4.1-distroless Docker container with Docker --read-only, then POSTing a key to eth/v1/keystores. It'll fail because /tmp is read-only, and produce a plaintext error message.