all: Tweak error creation (#6391)

- In the few places where we wrap errors, use the new Go 1.13 "%w"
  construction instead of %s or %v.

- Where we create errors with constant strings, consistently use
  errors.New and not fmt.Errorf.

- Remove capitalization from errors in the few places where we had that.

lib/connections/service.go

@@ -941,7 +941,7 @@ func (s *service) validateIdentity(c internalConn, expectedID protocol.DeviceID)
 	if remoteID == s.myID {
 		l.Infof("Connected to myself (%s) at %s - should not happen", remoteID, c)
 		c.Close()
-		return fmt.Errorf("connected to self")
+		return errors.New("connected to self")
 	}
 
 	// We should see the expected device ID