environs: move errors from errors package

environs: move errors from errors package

The errors package should be reserved for errors
that are widely applicable. Both "not bootstrapped"
and "container not supported" are environs-specific
and hence best defined there.

Along the way, we change the errors to be
more fit for purpose (no need for a nested error;
no need for a custom message in ErrNotBootstrapped).

BTW I'm not sure quite why we have a specific "containers
unsupported" error, but I assume it will support
some functionality as yet to arrive.

https://code.launchpad.net/~rogpeppe/juju-core/423-move-errors/+merge/187990

(do not edit description out of merge proposal)

[rog, 2013-09-27 08:24:10]

Please take a look.

[axw1, 2013-09-27 08:30:07]

On 2013/09/27 08:24:10, rog wrote:
> Please take a look.

LGTM

[mue, 2013-09-27 08:46:27]

LGTM, nice cleanup. Still not happy about the mix of own error types and using
of errors.New or fmt.Errorf, but that's not to be discussed here.