SDK error revamp (#432)

SDK methods now return 1 error.  To differentiate between Client errors and Server errors I've also implemented the AsServerError function which returns an *api.Error if it is an error originating from the remote server or nil otherwise.

api/authmethods/authenticate.go

@@ -4,13 +4,12 @@ import (
 	"context"
 	"fmt"
 
-	"github.com/hashicorp/boundary/api"
 	"github.com/hashicorp/boundary/api/authtokens"
 )
 
-func (c *Client) Authenticate(ctx context.Context, authMethodId string, credentials map[string]interface{}, opt ...Option) (*authtokens.AuthTokenReadResult, *api.Error, error) {
+func (c *Client) Authenticate(ctx context.Context, authMethodId string, credentials map[string]interface{}, opt ...Option) (*authtokens.AuthTokenReadResult, error) {
 	if c.client == nil {
-		return nil, nil, fmt.Errorf("nil client in Authenticate request")
+		return nil, fmt.Errorf("nil client in Authenticate request")
 	}
 
 	_, apiOpts := getOpts(opt...)
@@ -21,20 +20,23 @@ func (c *Client) Authenticate(ctx context.Context, authMethodId string, credenti
 
 	req, err := c.client.NewRequest(ctx, "POST", fmt.Sprintf("auth-methods/%s:authenticate", authMethodId), reqBody, apiOpts...)
 	if err != nil {
-		return nil, nil, fmt.Errorf("error creating Authenticate request: %w", err)
+		return nil, fmt.Errorf("error creating Authenticate request: %w", err)
 	}
 
 	resp, err := c.client.Do(req)
 	if err != nil {
-		return nil, nil, fmt.Errorf("error performing client request during Authenticate call: %w", err)
+		return nil, fmt.Errorf("error performing client request during Authenticate call: %w", err)
 	}
 
 	target := new(authtokens.AuthTokenReadResult)
 	target.Item = new(authtokens.AuthToken)
 	apiErr, err := resp.Decode(target.Item)
 	if err != nil {
-		return nil, nil, fmt.Errorf("error decoding Authenticate response: %w", err)
+		return nil, fmt.Errorf("error decoding Authenticate response: %w", err)
+	}
+	if apiErr != nil {
+		return nil, apiErr
 	}
 
-	return target, apiErr, nil
+	return target, nil
 }