Error converted to warning lost it's $fixable flag

Error reported by sniff, that is transformed to warning (according to severity override from ruleset.xml), wasn't retaining it's $fixable flag passed on from sniff itself.

In fact it should have raised Fatal Error:
![phpcs_file_adderror_fatalerror](https://cloud.githubusercontent.com/assets/1277526/4111616/4a27da98-320d-11e4-969c-105db40a62a1.png)

I guess such case isn't covered by any test.