Added Seam for custom exceptions

If you want to customize what error is thrown (#596) , you can now do the following:

// global 
private final Closeable oldGenerator = Approvals.settings().registerErrorGenerator((received, approved) -> new AssertionError("Custom message")))

try (AutoCloseable old = Approvals.settings()
    .registerErrorGenerator((received, approved) -> new AssertionError("Custom message")))
{
 ...
}