Replace DAVError with HTTPError + Error

That way we can avoid having different ways of representing the
same error value.

caldav/server.go

@@ -327,14 +327,10 @@ const (
 func NewPreconditionError(err PreconditionType) error {
 	name := xml.Name{"urn:ietf:params:xml:ns:caldav", string(err)}
 	elem := internal.NewRawXMLElement(name, nil, nil)
-	e := internal.Error{
-		Raw: []internal.RawXMLValue{
-			*elem,
+	return &internal.HTTPError{
+		Code: 409,
+		Err: &internal.Error{
+			Raw: []internal.RawXMLValue{*elem},
 		},
 	}
-	return &internal.DAVError{
-		Code: 409,
-		Msg:  fmt.Sprintf("precondition not met: %s", string(err)),
-		Err:  e,
-	}
 }

carddav/server.go

@@ -469,14 +469,10 @@ const (
 func NewPreconditionError(err PreconditionType) error {
 	name := xml.Name{"urn:ietf:params:xml:ns:carddav", string(err)}
 	elem := internal.NewRawXMLElement(name, nil, nil)
-	e := internal.Error{
-		Raw: []internal.RawXMLValue{
-			*elem,
+	return &internal.HTTPError{
+		Code: 409,
+		Err: &internal.Error{
+			Raw: []internal.RawXMLValue{*elem},
 		},
 	}
-	return &internal.DAVError{
-		Code: 409,
-		Msg:  fmt.Sprintf("precondition not met: %s", string(err)),
-		Err:  e,
-	}
 }

internal/internal.go

@@ -103,14 +103,3 @@ func (err *HTTPError) Error() string {
 func (err *HTTPError) Unwrap() error {
 	return err.Err
 }
-
-// DAVError is a XML error with HTTP status and a human readable message
-type DAVError struct {
-	Code int
-	Msg  string
-	Err  Error
-}
-
-func (err *DAVError) Error() string {
-	return err.Msg
-}

internal/server.go

@@ -2,6 +2,7 @@ package internal
 
 import (
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"mime"
 	"net/http"
@@ -10,16 +11,19 @@ import (
 )
 
 func ServeError(w http.ResponseWriter, err error) {
-	if davErr, ok := err.(*DAVError); ok {
-		w.WriteHeader(davErr.Code)
-		ServeXML(w).Encode(davErr.Err)
+	code := http.StatusInternalServerError
+	var httpErr *HTTPError
+	if errors.As(err, &httpErr) {
+		code = httpErr.Code
+	}
+
+	var errElt *Error
+	if errors.As(err, &errElt) {
+		w.WriteHeader(code)
+		ServeXML(w).Encode(errElt)
 		return
 	}
 
-	code := http.StatusInternalServerError
-	if httpErr, ok := err.(*HTTPError); ok {
-		code = httpErr.Code
-	}
 	http.Error(w, err.Error(), code)
 }