Protect against nil being passed to [SHK error:]

It appears that the read it later sharer returns an error code sometimes, but doesn't set the `X-Error` header leading to nil being passed to `[SHK error:]`. This causes the crash shown in the image below.

![crash log](http://f.cl.ly/items/1v1t0T2x2h2W0i0I0A00/Screen_Shot_2013-02-22_at_10.07.09_PM.png)