Change ExtLoader to only handle I/O errors

Change ExtLoader to only handle I/O errors

Due to the Control.Exception changes between 6.8 and 6.10, using it
portably is difficult. Since we're only interested in handling I/O
errors, we can use prelude's catch and not have to deal with
Control.Exception at all.

The use in Luxi.hs where we just use bracket is fine across the two
versions, so we keep that.