Meaningful info on PG::ActiveSqlTransaction error [WIP]

Currently when performing sql queries that cannot be run in transaction in migrations, like `create index concurrently` in postgresql, error message is not very meaningful. This is my try on adding better hint to make life of RoR newbies little bit easier.

This is marked as WIP for 2 reasons:
1. This is postgresql only so far.
2. I am not sure about design of this solution. I tried to do it in cleaner way, but this is what I came up with so far.

I would appreciate any help on this :heart:.