[API] Error json responses must be standartized

[Tronerta]

Error response during oauth process:

{
    "error": "invalid_client",
    "error_description": "Client authentication failed due to unknown client, no client authentication included, or unsupported authentication method."
}

Errors everywhere else:

{
    "error": [
        "Email has already been taken", "Trip must exist"
    ]
}

[zlacheuski]

Add error_code to response, rename error --> error_message

[Tronerta]

Done