Handle exception from find_and_modify with empty collection

[bookwyrm]

Catch and "handle" the exception that Mongo now returns (and Monped propagates) when you try to find_and_modify on an empty collection.

[ndelage]

+1 This fix patch was necessary for me to get delayed_job working with the latest version of mongoid (3.0.5)
and moped (1.2.0). Thanks @bookwyrm

[sferik]

A few things before we can merge this patch:

1. If you reference the Moped constant, it should be explicitly required at the top of the file, as well as in the gemspec.
2. It seems quite brittle to depend on the exact string 'No matching object found'. Are there any guarantees this string won't change in the future? Is there anything more reliable we can check for?
3. Please include at least two tests with this patch, one for the case where nil is returned and one for when an error is raised.

[dblock]

I believe this is no longer necessary, but if anyone thinks otherwise please reopen/rebase/write a test.