Export ServeJSON for serving error codes

This changeset provides a common http handler for serving errcodes. This should
unify http responses across webservices in the face of errors.

Several type assertions have been added, as well, to ensure the error interface
is implemented.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/api/errcode/handler.go

@@ -0,0 +1,44 @@
+package errcode
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// ServeJSON attempts to serve the errcode in a JSON envelope. It marshals err
+// and sets the content-type header to 'application/json'. It will handle
+// ErrorCoder and Errors, and if necessary will create an envelope.
+func ServeJSON(w http.ResponseWriter, err error) error {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	var sc int
+
+	switch errs := err.(type) {
+	case Errors:
+		if len(errs) < 1 {
+			break
+		}
+
+		if err, ok := errs[0].(ErrorCoder); ok {
+			sc = err.ErrorCode().Descriptor().HTTPStatusCode
+		}
+	case ErrorCoder:
+		sc = errs.ErrorCode().Descriptor().HTTPStatusCode
+		err = Errors{err} // create an envelope.
+	default:
+		// We just have an unhandled error type, so just place in an envelope
+		// and move along.
+		err = Errors{err}
+	}
+
+	if sc == 0 {
+		sc = http.StatusInternalServerError
+	}
+
+	w.WriteHeader(sc)
+
+	if err := json.NewEncoder(w).Encode(err); err != nil {
+		return err
+	}
+
+	return nil
+}