Improve formatting of ActiveRecord exception messages

Two changes to AR exception messages, with the primary motivation to make them more readable in the console.

1. for all messages, add two newlines above and two new newlines below
2. for the "you forgot to migrate" message (happens to me daily within guard..), make the command to fix the problem easier to find and copy.

What do you think?

specs: I didn't find any related specs and the sqlite specs all passed. Let me know if these changes should have spec changes.

With an 80-column terminal...

### before
![image](https://f.cloud.github.com/assets/6097/1285871/e4fcf556-2fb8-11e3-8c7c-89750cdeb3d1.png)

### after
![image](https://f.cloud.github.com/assets/6097/1285874/007e221e-2fb9-11e3-948c-70f875f1593f.png)