Add errgo as a dep, and start using it.

Add errgo as a dep, and start using it.

errgo adds the ability to capture the location of
the error creation or annotation. Annotated errors
don't lose the original error.

https://code.launchpad.net/~thumper/juju-core/use-errgo/+merge/204604

(do not edit description out of merge proposal)

[thumper, 2014-02-04 06:38:32]

Please take a look.

[william.reade, 2014-02-04 07:46:29]

LGTM

[rog, 2014-02-04 13:56:12]

https://codereview.appspot.com/59930043/diff/1/dependencies_test.go
File dependencies_test.go (right):

https://codereview.appspot.com/59930043/diff/1/dependencies_test.go#newcode30
dependencies_test.go:30: func (*dependenciesTest) TestDependenciesTsvFormat(c
*gc.C) {
I think this would work work better as a test in .lbox.check.
There is no package in the root directory and this doesn't really seem like a
great reason to create one.

awk -F "\t" 'NF != 4 {exit 1}' || {
    echo dependencies.tsv is in an invalid format >&2
    exit 1
}

would do the job.