Sometimes it is necessary to raise an exception. When you do this, you should always include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the built in error types, but should still include a meaningful message.
This particular exercise requires that you use the raise statement to "throw" multiple ValueErrors if the Tree() class is passed a tree that cannot be reoriented, or a path cannot be found between a start node and an end node. The tests will only pass if you both raise the exception and include a message with it.
To raise a ValueError with a message, write the message as an argument to the exception type:
# when a tree cannot be oriented to a new node POV
raise ValueError("Tree could not be reoriented")
#when a path cannot be found between a start and end node on the tree.
raise ValueError("No path found")