Make cabal-install more resilient to incorrect cabal files in index

The most recent incarnation of this is:

https://github.com/haskell/hackage-server/issues/351

The problem there was a byte order marker (BOM) added to the front of the cabal file, which wasn't parsed (I'll open a separate issue about that). However, this raises an issue we've seen with cabal in the past: a single problematic file will block cabal-install from doing some activities until it is resolved. Instead, it seems like a more reasonable behavior would be:
- Print a warning about that file
- Ignore that file completely

As it stands, an attacker could detect a bug in Hackage Server and continually exploit it with broken cabal files until Hackage is fixed.