Define separate error types for activation service errors

[sameh-farouk]

Description

EnsureAccount method now returns a custom error type ActivationServiceError for activation service-related errors.
This allows for easily identifying the source of the error by checking the type of error returned.

for example:

info, err := s.EnsureAccount(identity, activationURL, termsAndConditionsLink, terminsAndConditionsHash)
if err != nil {
    switch errType := err.(type) {
    case ActivationServiceError:
        // Handle Activation Service error
        fmt.Println("Activation service error:", errType.Err)
    default:
        // Handle other error types
        fmt.Println("Substrate error:", err)
    }
}

Related Issues:

• go-client: Add the ability to distinguish errors in EnsureAccount #985

Checklist:

• I have added tests to cover my changes.
• My commits follow this conventional commits guide.