Don't swallow exceptions in transctional statements

The MySQL connection adapater swallows all StandardError exceptions, which includes ```Mysql::Error``` and ```Mysql2::Error```. The comment in the exception clause claims errors thrown here indicate that transactions aren't supported by the server but that isn't necessarily true. It's possible the MySQL server has gone away and swallowing a failed commit may let the application return a successful response when the data has not been saved. Also, replication libraries like Galera require that the application handle exceptions thrown at BEGIN/COMMIT.

I'm unable to determine what version of MySQL threw an exception for transactional statements. I tried as far back as 3.23.49 with InnoDB disabled but BEGIN & COMMIT statements do not throw an error. If there's a real case for this logic to continue, we could instead push this behavior into a configuration setting.

The exception swallowing has been there since the beginning: db045dbbf60b53dbe013ef25554fd013baf88134

Backstory
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I found this behavior while moving our application to Percona XtraDB Cluster, which uses Galera for synchronous replication. We found a case where a transaction failed the certification process, and therefore threw an exception when COMMIT was called, but the exception was swallowed and the response to the client was an erroneous 200.