Implement error checking from `make_edge()`

[EwanC]

Our graphs specification defines the following conditions deriving from make_edge()

• queue is recording commands to the graph object.
• if src or dest are not valid nodes assigned to the graph object.
• if src and dest are the same node.
• cycle discovered on finalization.

We should implement checks for these in the code, and test the exceptions are thrown.

[EwanC]

Make edge parameters names also need updated in the prototype to match change in #40