New checker "jc.IsNil"

[Tim Penhey]

'ello 'ackers,

This is something that I have been meaning to add for quite some time.

We have been recording stack traces with the juju/errors library, but
until now, they have been of limited use.

One of the key places I have wanted to see the stack trace for a while
is when running tests, and I have a function that returns an error, I'd
normally have:

  c.Assert(err, gc.IsNil)

to make sure the error is nil.  Now I have added a checker into the
juju/testing/checkers that is error aware.

If you now use jc.IsNil instead of gc.IsNil, and the value is an error,
and the error has a stack trace, it will get printed out.

Very handy.

The dependencies of master have the versions you need to use it now.

Cheers,
Tim