Add support for customisation of error response payload

[tobiemh]

Added support for customisation of error responses using a custom error handler function. This allows to customise the format and structure of all error messages generated by go-json-rest, if needed.

This should be more versatile than the pull request suggested in #174, and does not add another method which would probably go unused in most implementations.

Example 1 (inline method)

package main

import (
    "github.com/ant0ine/go-json-rest/rest"
    "log"
    "net/http"
)

func main() {
    api := rest.NewApi()
    rest.ErrorHandler = func(w rest.ResponseWriter, code int) {
        w.WriteHeader(code)
        w.WriteJson(code)
    }
    router, err := rest.MakeRouter(
        rest.Trace("/", func(w rest.ResponseWriter, req *rest.Request) {
            w.WriteJson("Trace method called")
        }),
    )
    if err != nil {
        log.Fatal(err)
    }
    api.SetApp(router)
    log.Fatal(http.ListenAndServe(":8080", api.MakeHandler()))
}

func errors(w api.ResponseWriter, code int) {
    w.WriteHeader(code)
    w.WriteJson(code)
}

Example 2 (extensive configuration)

package main

import (
    "github.com/ant0ine/go-json-rest/rest"
    "log"
    "net/http"
)

func main() {
    api := rest.NewApi()
    rest.ErrorHandler = errors
    router, err := rest.MakeRouter(
        rest.Trace("/", func(w rest.ResponseWriter, req *rest.Request) {
            w.WriteJson("Trace method called")
        }),
    )
    if err != nil {
        log.Fatal(err)
    }
    api.SetApp(router)
    log.Fatal(http.ListenAndServe(":8080", api.MakeHandler()))
}

package main

import (
    "github.com/ant0ine/go-json-rest/rest"
)

func errors(w rest.ResponseWriter, code int) {
    w.WriteHeader(code)
    w.WriteJson(errs[code])
}

var errs = map[int]interface{}{

    404: map[string]interface{}{
        "code":          404,
        "details":       "Request resource not found",
        "information":   "The requested resource does not exist. Check that you have entered the url correctly.",
    },

    405: map[string]interface{}{
        "code":          405,
        "details":       "This method is not allowed",
        "information":   "The requested http method is not allowed for this resource. Refer to the documentation for allowed methods.",
    },

    415: map[string]interface{}{
        "code":          415,
        "details":       "Unsupported content type requested",
        "information":   "Requests to the api must use the 'Content-Type: application/json' header. Check your request settings and try again.",
    },

    500: map[string]interface{}{
        "code":          500,
        "details":       "There was a problem with our servers, and we have been notified",
    },

}

[ant0ine]

Hi, thanks for the pull request.

If ErrorHandler is defined and the user calls rest.Error(...) withe an error message, then the error message is ignored in favor of the custom handler. This is a confusing API.

I'm not convinced that this is worth the added complexity. If the default Error handler is not good enough for a particular use case, then just write another one.