Should `DigraphRemoveEdge{s}` and `DigraphRemoveVertex/ices` should give an error when asked to remove edges/vertices that don't exist?

[wilfwilson]

This question was raised in #260, specifically in @saffronmciver's comment #260 (comment).

What does everyone think? I think that either

Whatever we choose, we should be consistent, and make sure that the behaviour of all analogous functions is either consistent in giving an error, or consistent about either returning the original digraph unchanged, or returning a new equal digraph.

[james-d-mitchell]

I think we should give an error, good point @wilfwilson, and @saffronmciver. If no one disagrees, then @devansh2605 will add these checks!