add some function to adjust juju/error

[WangXiangUSTC]

IsNotFound
NotValidf
IsAlreadyExists
AlreadyExistsf

[WangXiangUSTC]

@gregwebs PTAL

[gregwebs]

This is good from a compatibility perspective. However, I would like to move towards using the pkg/errors API and removing the need for the juju adapater if possible. That includes using structured errors rather than string matching.

pkg/errors has a function that can be used to find a structured error type rather than just string matching.

errcode uses this and formalizes error types as error codes.

So I am also wondering which projects need this and if they can start to consider using proper error types.

But we can also merge this now and put off refactoring until later.

[WangXiangUSTC]

I agree with you, this is just a temporary solution, because I need use these function now, it is a little emergency, we can merge this now, and refine it later. @gregwebs

[gregwebs]

I think we should make the juju adapter be a separate package.

[WangXiangUSTC]

@gregwebs please merge this pr, I am not authorized to do this, thanks!

[lysu]

LGTM... I don't have permission too

[gregwebs]

merged. @lysu that's weird: you merged PRs in the past and I don't think permissions changed. I made you an admin if you want to look at the settings.