include line/char info on parse errors

This PR adds information to parse errors, to make it easier to (for example) provide feedback within template editors (which is the use case that prompted this - improving the editor in learn.ractivejs.org).

Specifically, the error object now has three additional properties - `line`, `character` (both 1-based rather than 0-based), and `shortMessage`, which is the error message stripped of the `at line x character y ... ^---` annotation.