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.