Strictly speaking this breaks Haskell 98 compatibility, which requires
hGetContents to just end the lazy stream silently if an error is
encountered.  However, for a few reasons we think it will make
everyone's life a bit easier if we make this change

 1. Errors will be a lot more common in GHC 6.12.1, in the form
    of Unicode decoding errors.

 2. When Haskell 98 was designed, we didn't know how to throw
    exceptions from inside lazy I/O, but now we do.

 3. If anyone is actually relying on the previous behaviour, their
    code is arguably broken.