all: Make all error implementations pointer types (#6726)

This matches the convention of the stdlib and avoids ambiguity: when
customErr{} and &customErr{} both implement error, client code needs to
check for both.

Memory use should remain the same, since storing a non-pointer type in
an interface value still copies the value to the heap.

lib/upnp/upnp.go

@@ -79,7 +79,7 @@ type UnsupportedDeviceTypeError struct {
 	deviceType string
 }
 
-func (e UnsupportedDeviceTypeError) Error() string {
+func (e *UnsupportedDeviceTypeError) Error() string {
 	return fmt.Sprintf("Unsupported UPnP device of type %s", e.deviceType)
 }