go-client: Add the ability to distinguish errors in `EnsureAccount`

[AbdelrahmanElawady]

Is your feature request related to a problem? Please describe

When using the go-client there is no way to determine whether the error is related to tfchain or the activation service is down in EnsureAccount. Which makes it difficult to decide whether to retry another activation service or abort.

Describe the solution you'd like

Maybe add an exported error to indicate the error is due to activation service being down.