Try to avoid exposing third-party exceptions

[clbarnes]

Instead, wrap them in a transformnd exception.

class TransformndException(Exception):
    ...

class TransformndNoPath(TransformndException):
    ...


try:
    seq = graph.get_sequence(B, A)
except networkx.exception.NetworkXNoPath as e:
    raise TransformndNoPath(f"no path between {B} and {A}") from e

Raised by @Tomaz-Vieira here #74