http3: introduce an HTTP/3 error type (#4039)

* http3: introduce an HTTP/3 error type

* http3: use a pointer receiver for the Error

http3/error.go

@@ -0,0 +1,58 @@
+package http3
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/quic-go/quic-go"
+)
+
+// Error is returned from the round tripper (for HTTP clients)
+// and inside the HTTP handler (for HTTP servers) if an HTTP/3 error occurs.
+// See section 8 of RFC 9114.
+type Error struct {
+	Remote       bool
+	ErrorCode    ErrCode
+	ErrorMessage string
+}
+
+var _ error = &Error{}
+
+func (e *Error) Error() string {
+	s := e.ErrorCode.string()
+	if s == "" {
+		s = fmt.Sprintf("H3 error (%#x)", uint64(e.ErrorCode))
+	}
+	// Usually errors are remote. Only make it explicit for local errors.
+	if !e.Remote {
+		s += " (local)"
+	}
+	if e.ErrorMessage != "" {
+		s += ": " + e.ErrorMessage
+	}
+	return s
+}
+
+func maybeReplaceError(err error) error {
+	if err == nil {
+		return nil
+	}
+
+	var (
+		e      Error
+		strErr *quic.StreamError
+		appErr *quic.ApplicationError
+	)
+	switch {
+	default:
+		return err
+	case errors.As(err, &strErr):
+		e.Remote = strErr.Remote
+		e.ErrorCode = ErrCode(strErr.ErrorCode)
+	case errors.As(err, &appErr):
+		e.Remote = appErr.Remote
+		e.ErrorCode = ErrCode(appErr.ErrorCode)
+		e.ErrorMessage = appErr.ErrorMessage
+	}
+	return &e
+}