Issue 3270

Resolves issue: http://code.google.com/p/otwarchive/issues/detail?id=3270

The series controller now checks to ensure that the series the user is trying to access actually exists. If it does not, the user is sent back a page, or to the works page (the most likely place they came from?) and given a meaningful error message.
Presumably you can do the ```find_by_id``` only once, and store it in ```@series```, then check if it's nil or not and redirect or not accordingly, rather than retrieving it once by ```find_by_id``` and then by ```find```. Also, indentation is missing.