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.
Could you also an example that doesn't throw an exception and uses a `Callable<something>' (where "something" isn't 'Object') and verify that the return value is the same as the passed-in value?