state/..., worker/..., utils/...: use errgo

state/..., worker/..., utils/...: use errgo

This is an example of what things look like when
changed wholesale to use an error wrapping package.
I realise that no decision has been made yet, but
this shows how a significant proportion of the code
would look when using errgo.

Most of the changes were made automatically;
all calls to wrap with extra arguments were changed
manually (that ended up at around 10% of all calls).

https://code.launchpad.net/~rogpeppe/juju-core/500-errgo-fix/+merge/206024

(do not edit description out of merge proposal)