Don't throw exceptions from top-level bindings, even if we can't talk to xen

Instead we raise the exception when the user calls {wait,after}, i.e.
when they actually want to use event channels. The user application can
catch the Failure exception and perform some diagnostics.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #7 from djs55/dont-fail-on-startup

Dont fail on startup