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.

internal/discover/discover.go

@@ -132,9 +132,9 @@ func (d *Discoverer) ExtAnnounceOK() bool {
 }
 
 func (d *Discoverer) Lookup(device protocol.DeviceID) []string {
-	d.registryLock.Lock()
+	d.registryLock.RLock()
 	cached := d.filterCached(d.registry[device])
-	d.registryLock.Unlock()
+	d.registryLock.RUnlock()
 
 	if len(cached) > 0 {
 		addrs := make([]string, len(cached))