Editing the error message for the sampling_zero_paths argument

[ashenvi10]

In the EventTree class, the sampling_zero_paths argument error message should be changed to reflect the following:
[ ] You do not, in fact, need to end the tuple with a comma for it to work. This what the current message says and we should remove this.
[ ] The elements in the tuple for the path should match exactly (they are case sensitive) with the edge labels in the tree. The error message should ask the user to confirm this.