Improve error message for addE() when traverser is incorrect

For this:

gremlin> g.addV('A').as('a').addE('self').addE('self2').to('a')
addE(self2) could not find a Vertex for from() - encountered: TinkerEdge
Type ':help' or ':h' for help.
Display stack trace? [yN]n

The traverser for the second addE('self2') is not a Vertex` - it is the previously added Edge of addE('self'). Since `from()` is not provided for the second addE('self2') it uses the current traverser and we get the exception shown. The exception is a bit confusing for those who don't realize what it means. Change the exception to: "addE(self2) could not find a Vertex specified for from() - a TinkerEdge was specified instead" - or perhaps a better message?