Improvement: Tell me where I am in the tree when a parse failure happens

[jasminemoore-verses]

The error message currently includes which AST (struct|enum) is currently being parsed, but does not say which Pest token we are currently at.

• "Extraneous tokens" should tell me which tokens are bonus
• "No Match" should tell me what token is present.

[oxcrow]

This would be extremely helpful.

As I've seen, pest prints helpful errors, and hopefully pest-ast can also print similarly helpful errors.