Support multi registrations for custom http errors

[kataras]

The problem

The problem is with app.OnErrorCode(404, ... we can register only one handler, that issue can be solved by wrapping handlers together, it's basic functional way to do it, but then we have second problem

Middleware, the second problem

Today we can add middleware to custom errors by calling them manually inside our custom handler, i.,e func(ctx context.Context){ myErrMiddlewre(ctx); myErrHandler(ctx) }.

The ctx.Next() inside these handlers will not be managed correctly, so wrapping handlers for these kind of things is not a solution for apps who depends on middlewares that are specific registered to error handlers.

Solution

Both issues can be solved by allowing registration of multiple Handlers for a specific http error.
i.e app.OnErrorCode(404, myErrMiddleware, myErrHandler) , the ctx.Next() will be respected, so if the myErrMiddleware isn't calling the ctx.Next() then myErrHandler will be not executed, as expected.

Completed

With fe256fe