Improve missing-module error messages.  eg.

hello2.hs:1:
    Failed to load interface for `Foo':
        Could not find interface file for `Foo'
        (use -v to see a list of the files searched for)

and if we add the -v flag:

hello2.hs:1:
    Failed to load interface for `Foo':
        Could not find interface file for `Foo'
        locations searched:
            Foo.hi
            Foo.hi-boot-6
            Foo.hi-boot
            /home/simonmar/builds/i386-unknown-linux/libraries/haskell-src/Foo.hi
            /home/simonmar/builds/i386-unknown-linux/libraries/haskell98/Foo.hi
            /home/simonmar/builds/i386-unknown-linux/libraries/readline/Foo.hi
            /home/simonmar/builds/i386-unknown-linux/libraries/unix/Foo.hi
            /home/simonmar/builds/i386-unknown-linux/libraries/network/Foo.hi
            /home/simonmar/builds/i386-unknown-linux/libraries/base/Foo.hi