eth/downloader: more context in errors (#21067)

This PR makes use of go 1.13 error handling, wrapping errors and using
errors.Is to check a wrapped root-cause. It also removes the travis
builders for go 1.11 and go 1.12.

eth/downloader/queue.go

@@ -509,7 +509,7 @@ func (q *queue) reserveHeaders(p *peerConnection, count int, taskPool map[common
 		index := int(header.Number.Int64() - int64(q.resultOffset))
 		if index >= len(q.resultCache) || index < 0 {
 			common.Report("index allocation went beyond available resultCache space")
-			return nil, false, errInvalidChain
+			return nil, false, fmt.Errorf("%w: index allocation went beyond available resultCache space", errInvalidChain)
 		}
 		if q.resultCache[index] == nil {
 			components := 1
@@ -863,14 +863,16 @@ func (q *queue) deliver(id string, taskPool map[common.Hash]*types.Header, taskQ
 		q.active.Signal()
 	}
 	// If none of the data was good, it's a stale delivery
-	switch {
-	case failure == nil || failure == errInvalidChain:
-		return accepted, failure
-	case useful:
-		return accepted, fmt.Errorf("partial failure: %v", failure)
-	default:
-		return accepted, errStaleDelivery
+	if failure == nil {
+		return accepted, nil
 	}
+	if errors.Is(failure, errInvalidChain) {
+		return accepted, failure
+	}
+	if useful {
+		return accepted, fmt.Errorf("partial failure: %v", failure)
+	}
+	return accepted, fmt.Errorf("%w: %v", failure, errStaleDelivery)
 }
 
 // Prepare configures the result cache to allow accepting and caching inbound