Changes a handful of things in enumeration error paths.

Bugfixes:
  - consistent cleanup, and consistent return of -Ewhatever/0;
  - a usb_put_dev() gets rid of the extra refcount; this
    one's in bugzilla at osdl.

Diagnostics:
  - new diagnostic for when there's a choice of configurations;
  - merges some diagnostics, for slight rodata reduction.