Added json encoded for ServerErrors

Added json encoded for ServerErrors

added jsonEncode function to novaservice that
tries to convert the given error into a 
ServerError and call its AsJSON method or
wrap the given error into a 500 ServerError
which will contain the original error message.

https://code.launchpad.net/~hduran-8/goose/json_encoded_errors/+merge/217860

Requires: https://code.launchpad.net/~hduran-8/goose/testservice_errors/+merge/217818

(do not edit description out of merge proposal)

[hduran, 2014-05-01 01:09:14]

Please take a look.

[hduran, 2014-05-02 15:39:55]

Please take a look.

[gz, 2014-05-02 16:19:48]

LGTM.

https://codereview.appspot.com/97870044/diff/20001/http/client.go
File http/client.go (right):

https://codereview.appspot.com/97870044/diff/20001/http/client.go#newcode62
http/client.go:62: return nil, err
We seem to not be able to hit this error branch, that's fine.

https://codereview.appspot.com/97870044/diff/20001/http/client.go#newcode64
http/client.go:64: response.Title = key
Add break here. May as well exit early if we're passed something with many top
level keys.

https://codereview.appspot.com/97870044/diff/20001/http/client.go#newcode69
http/client.go:69: return nil, fmt.Errorf("Unexpected response format: %q",
jsonBytes)
"Unparsable json error body"

https://codereview.appspot.com/97870044/diff/20001/http/client_test.go
File http/client_test.go (right):

https://codereview.appspot.com/97870044/diff/20001/http/client_test.go#newcod...
http/client_test.go:191: c.Assert(unmarshalled.Message, Equals, "A Meaningful
error")
These attribute compares should be Check not Assert.

[hduran, 2014-05-02 16:29:34]

Please take a look.