[plt-scheme] exception conventions

Update to my earlier example: I decided to expose the exception type
hierarchy in the names, and to otherwise abbreviate them:

  (define-struct (exn:httper                 exn)              ())
  (define-struct (exn:httper:proto           exn:httper)       ())
  (define-struct (exn:httper:proto:http      exn:httper:proto) (detail))
  (define-struct (exn:httper:proto:redirects exn:httper:proto) ())
  (define-struct (exn:httper:proto:transport exn:httper:proto) (sub-exn))
  (define-struct (exn:httper:status          exn:httper)       (code reason))
  (define-struct (exn:httper:uri             exn:httper)       (uri))

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