Use generics for DepSets

Use Go's generics for DepSets so they don't require a type-specific
wrapper and reflection.

Test: depsets_test.go
Change-Id: I22ba0b7d680d37d2cd05230b0f560d166c4dd20b
diff --git a/ui/metrics/metrics.go b/ui/metrics/metrics.go
index 6d60316..d68ced8 100644
--- a/ui/metrics/metrics.go
+++ b/ui/metrics/metrics.go
@@ -230,7 +230,7 @@
 
 func (m *Metrics) UpdateTotalRealTimeAndNonZeroExit(data []byte, bazelExitCode int32) error {
 	if err := proto.Unmarshal(data, &m.metrics); err != nil {
-		return fmt.Errorf("Failed to unmarshal proto", err)
+		return fmt.Errorf("Failed to unmarshal proto: %w", err)
 	}
 	startTime := *m.metrics.Total.StartTime
 	endTime := uint64(time.Now().UnixNano())