return a net.Error when opening streams

net.Error.Temporary() will be true if no stream can be opened when the
peer's stream limit is reached.

streams_map.go

@@ -1,7 +1,9 @@
 package quic
 
 import (
+	"errors"
 	"fmt"
+	"net"
 
 	"github.com/lucas-clemente/quic-go/internal/flowcontrol"
 	"github.com/lucas-clemente/quic-go/internal/handshake"
@@ -9,6 +11,16 @@ import (
 	"github.com/lucas-clemente/quic-go/internal/wire"
 )
 
+type streamOpenErr struct{ error }
+
+var _ net.Error = &streamOpenErr{}
+
+func (e streamOpenErr) Temporary() bool { return e.error == errTooManyOpenStreams }
+func (streamOpenErr) Timeout() bool     { return false }
+
+// errTooManyOpenStreams is used internally by the outgoing streams maps.
+var errTooManyOpenStreams = errors.New("too many open streams")
+
 type streamsMap struct {
 	perspective protocol.Perspective