[Bug]: `Pattern.get_layers` does not support non-runnable patterns

Consider the following pattern where the corrections define a closed loop:

```python
from graphix.pattern import Pattern
p1 = Pattern(input_nodes=[0, 1, 2], cmds=[X(0, domain={1}), X(1, domain={2}), X(2, domain={0})])
```

Calling `p1.get_layers()` results in an infinite loop. Desired behaviour should be to return `None` or raise an exception.

To be fixed in Flow refactor part 2.

Related #364 
