em.commit doesn't throw when a sql transaction commit fails

[rhyek]

Extracting this small issue out of #1160 and providing fix via PR.

Basically, when using explicit transactions and explicit begin, commit, and rollback methods, if the commit statement fails on the database side, this is not currently being caught by mikroorm and em.commit resolves incorrectly. It should rethrow the error given by the knex transaction context.