source-mysql-batch: Failures when trying to capture from SingleStore

[willdonnelly]

SingleStore is "MySQL wire protocol compatible" so in theory we should be able to capture from it. In practice it's failing with an error message saying:

unable to connect to database: <nil>

I suspect there are at least two issues here: one with the error reporting and one which is actually causing it to fail.

[willdonnelly]

I've confirmed that there are in fact three errors going on here:

• Incorrect variable shadowing in the error handling resulting in where there should have been a useful error message.
• If connecting with SSL fails we retry without SSL, which in this case fails because SSL is required. This means that the original error is yet again hidden from us.
• The actual original error in this case appears to be unable to connect to database: handleAuthResult: readAuthResult: ERROR 1046 (3D000): In order to use shared tier you need to select a database by default

[willdonnelly]

We've confirmed that the real "you need to select a database" error can be addressed by setting the /advanced/dbname property when connecting to SingleStore. Everything else is just some error reporting jank which I've got a PR out to fix.