Improve formatting of ActiveRecord migration exception messages

Two changes to AR exception messages during migrations. Rationale: the context in which these exceptions happen is very often when a migration is invoked by a human in a console session, either during development or when deploying. So, improving the readability is highly beneficial.

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.

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)