Switch to new-style exception handling

Currently, we're using Prelude.catch to handle I/O errors in
htools. This style of error handling has been deprecated for a while,
but it still used to work without warnings.

However, the GHC release 7.4 has started to emit deprecation warnings
for it, so we change to the Control.Exception module; the code is a
bit less clean since we only care about I/O errors (but
Control.Exception deals with other error types too), so we have to
filter the exceptions.

Note that the new style exception handling is not really "new"; it has
existed since at least GHC 6.12, which is our oldest supported
compiler.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>