Description

This PR resolves issue #39, which is a case that gets hit if a tile request is canceled while we are still processing the BeginTx call.

In that case nil is returned for tx (and err is non-nil) and we crash as described in #39.

This PR fixes the issue by exiting runQuery if BeginTx returns an error and removing the call to tx.Rollback altogether.

Note that not calling tx.Rollback at all is OK because

At this time we already have a defered call to cancel the Context on which the transaction was created.
According to the documentation on database/sql.BeginTx: If the context is canceled, the sql package will roll back the transaction.

This implies that the transaction will always be rolled back without an explicit call to tx.Rollback.

Thanks to @audreyyku and @jerluc for their help with this!

StationA / tilenol