Fix up exception handling when reading an interface file, and make it
compile with 4.08.x again.

GhcExceptions weren't being caught by readIface, so an error when
reading an interface could be unintentionally fatal (errors should be
soft when reading the old interface file for the current module).
Also, the Interrupted exception should not be caught by readIface,
because we want ^C to behave as normal when reading interface files
(currently it causes an interface-file read error rather than
interrupting the whole compiler).

Some exception-related compatibility functions have been moved from
Util to Panic.