optimize the mysql connection state check logic

[RaymRaym]

First, nice project. I learned a lot about golnag web development and react.js from it. Thank you.
Next, when I do project initialization, I found the logic of mysql connection creation part is not complete. The sql.Open method only validate the arguments but it does not verify the cconnection. So only after you check the connection by calling db.Ping(), can you print "Successfully connected to MySQL database". Following is the comment from sql package source code.

// Open may just validate its arguments without creating a connection
// to the database. To verify that the data source name is valid, call
// Ping.