Add error codes and define errors with it

[lpoli]

Our error structure in "github.com/0chain/errors" is;

type Error struct{
	code string
	msg string
}

In gosdk we have different error implementation. Sometimes above structure is considered; other time thrown.Throw is considered and
sometimes errors.New or fmt.Errorf is considered.
This makes error handling difficult and error response are inconsistent as well.
Below is just a structure considering error structure from "github.com/0chain/errors". We can work with other structure but it should be
consistently used inside gosdk.

One case:
Say I requested some file refs from 6 blobbers(with 4:2 ratio) and 3 of them gave too_many_requests error. If only two had given too_many_requests error then anyway we could take consensus from other 4 blobbers and request is successful. But with three blobbers giving those errors, consensus would not be reached so error would be consensus_failed. The application that made this request should have been let know that it was too_many_requests error that caused consensus failure, so that it would wait for some time interval and re-request again.

We can define codes as constant values so function would use it. Later some function can check which error it is with errors.Is function with the errors defined below in var.

[lpoli]

@Sriep @dabasov @cnlangzi
Please comment on this.

[cnlangzi]

@Sriep @dabasov @cnlangzi Please comment on this.

old errors.Wrap works with many code. many things might be broken. but I am not sure, you have to test all of them.

errors.Thrown is working with go errors v2 in blobber only.

[cnlangzi]

please make all system_tests passed first. and talk to @moldis about zmobile

[lpoli]

@cnlangzi
This is just an initial draft.
I will continue if you guys approve it.

[dabasov]

Having distinct errors is a good idea. I like GOish style of managing it, just having constant errors and wrap them when needed. Using 0chain custom error is not a problem too, since it implements errors interface.