[Improvement] Return values of methods in `DatabaseGateway` could be useful.

[ScottCTD]

• DatabaseGateway.getUserData(String) could return null if the given user does not exist in the database.
• Now, the behavior of failing to find a user is throwing a RuntimeException, which is not very cool.
• We can generalize this as all methods should return something that is useful.
• For example, the getBillData method may return null if the table related to the given id is not in the database, instead of throwing a SQLException.

[Lei-Tin]

I have a small problem.

If we were to select users and entry, that is fine. We can return null if the username / entry_id in question cannot be found. As the query ResultSet will be empty.

However, for other "Getter" methods from the database, if we pass in a table ID that is not quite right, MySQL gives a SQLException right away, and there is no way to tell whether this SQLException is actually caused by a wrong table ID or simply a bad connection.

Similar result goes for createBillTable and methods that aren't specific to grab a single result.

Also, aside from this, in my next PR, I will also include a SQL Script to generate a sample dataset that we can use to test things. Stay tuned for that!