Refactor doc collection, block, and add custom error

Hi all,
I've moved some of the comment block parsing responsibilities from the DocBlockCollection to the DocumentBlock. It made things a bit cleaner. 

Additional I added an error class, and rescued inside the DocBuilder. This is because Display.error was being called inside the DocCollection, so there was a possibility of it system exiting if i used that class on its own.

Any feedback would be appreciated.
J