[rpc][tracers] return nil if block not exists

hmy/tracers/internal/tracers/block_tracer.js

@@ -286,7 +286,7 @@
 			type: sortType,
 			action: action,
 			result: call.error ? undefined : result,
-			error: call.error ? "revert" : undefined
+			error: call.error ? "Reverted" : undefined
 		};
 		for (var key in sorted) {
 			if (sorted[key] === undefined) {

rpc/tracerParity.go

@@ -17,7 +17,7 @@ type PublicParityTracerService struct {
 func (s *PublicParityTracerService) Block(ctx context.Context, number rpc.BlockNumber) (interface{}, error) {
 	block := s.hmy.BlockChain.GetBlockByNumber(uint64(number))
 	if block == nil {
-		return block, errors.New("block not exist")
+		return nil, nil
 	}
 	traceJs := "blockTracer"
 	results, err := s.hmy.TraceBlock(ctx, block, &hmy.TraceConfig{Tracer: &traceJs})