Feature #2597 -- fix cycle relationship notification format.

The SimpleGraph class was reporting duplicate data when printing cycles:

Notify[c]Notify[c] => Notify[d]
Notify[a]Notify[a] => Notify[b]

This was caused by throwing the array representation of the edge into a
string, rather than just the relationship data; we only care about the later,
so now we only emit that later and have the correct text in the error.