Fallback to the |Dummy| Pattern when we fail to parse a Shading Pattern (issue 5509)

Fixes #5509.

/cc @yurydelendik 
I wanted to avoid duplicating the following lines twice (hence why I choose to throw):
```javaScript
  UnsupportedManager.notify(UNSUPPORTED_FEATURES.shadingPattern);
  return new Shadings.Dummy();
```
The end result is the same, just less repetition. Also, this particular code path shouldn't be very hot in practice.