Catch Parslet parse errors and re-raise them as an Upmark error

* so users of upmark don't need to know anything about Parslet or its
  errors
lock upmark to parslet 1.4.0

* We use the Parslet::ParseFailed execption, which was removed in
  parslet > 1.4.0
* Once I update upmark to stop using Parslet::ParseFailed this can be
  relaxed