Use savepoints to allow rerun failed scripts

[technic]

When migration script contains a mistake, then part of commands get executed. The second attempt will rerun script from the beginning, which results in executing the commands for the second time. This behavior is wrong.

[Keats]

I guess we could wrap the SQL execution in transactions. People can already add it in the SQL file themselves but that does get repetitive and error prone.

I'll take a PR for it!

[technic]

I can do it for sqlite, but I don't want to test it with other backends :)