consider using pkg/errors instead juju/errors

[siddontang]

Now we use juju errors and use errors.Wrap to wrap the error for later getting error stack. It is not convenient because we have to use errors.Wrap in every error return.

Most of the time, we just want to know the error stack, so it is better to use pkg/errors instead. We can use "%+v" to get the error stack and no need to wrap error when return every time.

Refer https://github.com/pkg/errors

[zhexuany]

@siddontang Any updates about this proposal?

I created a PR about error handling several day ago. I believe it is will be better if we finish error handling and then do the improvement. Currently, some math operation does not return error code correctly i.e select pow(10, 700). We need investigate this.