Make hGetContents throw an exception if an error is encountered

Make hGetContents throw an exception if an error is encountered

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.