widen generics in ErrorCollector

This brings the signatures in line with others like Assert.assertThat.
Especially the checkSucceeds would be hardly usable in languages with target typing (Java8/Xtend) otherwise.
I think a better signature would be:

    public <V> V checkSucceeds(Callable<V> callable)

If this is the only change (other than the tests) could you update the commit message to be someone more specific? Perhaps "Make ErrorCollector.checkSucceeds() generic"