web: Add more data for jsonrpc responses. (#3296)

This change adds more richer error response
for JSON-RPC by interpreting object layer
errors to corresponding meaningful errors
for the web browser.

```go
&json2.Error{
   Message: "Bucket Name Invalid, Only lowercase letters, full stops, and numbers are allowed.",
}
```

Additionally this patch also allows PresignedGetObject()
to take expiry parameter to have variable expiry.

cmd/storage-rpc-server_test.go

@@ -40,7 +40,7 @@ func createTestStorageServer(t *testing.T) *testStorageRPCServer {
 		t.Fatalf("unable initialize config file, %s", err)
 	}
 
-	jwt, err := newJWT(defaultInterNodeJWTExpiry)
+	jwt, err := newJWT(defaultInterNodeJWTExpiry, serverConfig.GetCredential())
 	if err != nil {
 		t.Fatalf("unable to get new JWT, %s", err)
 	}