[Chore] Improve API response structure

[longnd]

Issue

The API responses do not follow any JSON-compliant standard such as https://jsonapi.org/. The endpoints do not return adequate error responses consistently. For instance,

• The API for registering an account will return validation errors

• API/v1/SearchController will result in a server error when the params is invalid, e.g. empty value for the search[file] param (no file is uploaded).