misc improvements around read-language (uses and docs)

- make a logger for drracket-related #lang problems
- change read-language's docs to indicate that it can return #f
- adjust a bunch of calls to read-language for the case that it returns #f
  with the goal of making that be the same as a get-info function that
  always returns the given default
- mention `read-language` in the guide section entitled
 "Source-Handling Configuration"