REST API: provide error messages in error responses

[edigaryev]

Resolves #53.

[fkorotkov]

Have you looked in gin's context built-in Error method? It seems you can attache errors and then in a middleware handle it. Do you think it's more idiomatic? On the other hand we also introduced the Responder abstraction and this PR embraces it. 🤔

[edigaryev]

On the other hand we also introduced the Responder abstraction and this PR embraces it.

That's it, by using Responder we get way more explicit return value/error propagation, which reduces potential errors and makes code clearer.

With ctx.Error() (or ctx.AbortWithError()) things will always be more complicated, as you have not forget about return.

[fkorotkov]

SGTM. Then what do you think of just having body as plain text and not JSON? Or at least renaming NewError to NewErrorResponse?

[edigaryev]

Then what do you think of just having body as plain text and not JSON?

I don't see why we'd want that. JSON will enable for better future extensibility, and parsing it is a no-brainer in most languages.

Or at least renaming NewError to NewErrorResponse?

See 32a93d3.