SwaggerError everywhere

[cyrylkuzmin]

This PR makes SwaggerError type as the only type of Errors in API (removes our custom Error type) In additional:

• extend SwaggerError by Details field that contains some raw errors (like ent: something not found);
• move all error messages into 1 place and use these vars in handlers instead of strings;
• move some validations from repo to handlers (to generate errors properly);
• some updates in int tests: the most of error messages should be checked;