Minor fixes.

- Bug fix - use .Errorf() instead of .Error():
  Before this change, an error message for failed tests in files/set_test.go
  wrongly tried to use format strings in a call to .Error()

- Change event constants to be of type EventType rather than untyped integers.

- Slightly relax the locking for registry cache lookups:
  For read access to Discover.registry we only need to take a read lock.

- Some cosmetic fixes.