Return HTTP response codes as typed errors

This allows to differentiate a dashboard not found from other errors, e.g.

```go
_, err = client.GetDatasource(ctx, dsID)

if errors.Is(err, sdk.ErrNotFound) {
    fmt.Fprintf(os.Stderr, "Creating new datasource %s (id=%d)\n", ds.Name, ds.ID)
}
```

Author: suhlig

rest-datasource.go

@@ -22,7 +22,14 @@ package sdk
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"net/http"
+)
+
+var (
+	ErrNotFound      = errors.New("not found")
+	ErrAlreadyExists = errors.New("already exists")
 )
 
 // GetAllDatasources gets all datasources.
@@ -56,10 +63,14 @@ func (r *Client) GetDatasource(ctx context.Context, id uint) (Datasource, error)
 	if raw, code, err = r.get(ctx, fmt.Sprintf("api/datasources/%d", id), nil); err != nil {
 		return ds, err
 	}
-	if code != 200 {
-		return ds, fmt.Errorf("HTTP error %d: returns %s", code, raw)
+	switch code {
+	case http.StatusOK:
+		err = json.Unmarshal(raw, &ds)
+	case http.StatusNotFound:
+		err = fmt.Errorf("data source with id %q %w", id, ErrNotFound)
+	default:
+		err = fmt.Errorf("HTTP error %d: returns %s", code, raw)
 	}
-	err = json.Unmarshal(raw, &ds)
 	return ds, err
 }
 
@@ -89,17 +100,23 @@ func (r *Client) CreateDatasource(ctx context.Context, ds Datasource) (StatusMes
 		raw  []byte
 		resp StatusMessage
 		err  error
+		code int
 	)
 	if raw, err = json.Marshal(ds); err != nil {
 		return StatusMessage{}, err
 	}
-	if raw, _, err = r.post(ctx, "api/datasources", nil, raw); err != nil {
+	if raw, code, err = r.post(ctx, "api/datasources", nil, raw); err != nil {
 		return StatusMessage{}, err
 	}
-	if err = json.Unmarshal(raw, &resp); err != nil {
-		return StatusMessage{}, err
+	switch code {
+	case http.StatusOK:
+		err = json.Unmarshal(raw, &resp)
+	case http.StatusConflict:
+		err = fmt.Errorf("data source with name %q %w", ds.Name, ErrAlreadyExists)
+	default:
+		err = fmt.Errorf("HTTP status code %d: returns %s", code, raw)
 	}
-	return resp, nil
+	return resp, err
 }
 
 // UpdateDatasource updates a datasource from data passed in argument.
@@ -109,17 +126,23 @@ func (r *Client) UpdateDatasource(ctx context.Context, ds Datasource) (StatusMes
 		raw  []byte
 		resp StatusMessage
 		err  error
+		code int
 	)
 	if raw, err = json.Marshal(ds); err != nil {
-		return StatusMessage{}, err
+		return resp, err
 	}
-	if raw, _, err = r.put(ctx, fmt.Sprintf("api/datasources/%d", ds.ID), nil, raw); err != nil {
-		return StatusMessage{}, err
+	if raw, code, err = r.put(ctx, fmt.Sprintf("api/datasources/%d", ds.ID), nil, raw); err != nil {
+		return resp, err
 	}
-	if err = json.Unmarshal(raw, &resp); err != nil {
-		return StatusMessage{}, err
+	switch code {
+	case http.StatusOK:
+		err = json.Unmarshal(raw, &resp)
+	case http.StatusNotFound:
+		err = fmt.Errorf("data source with name %q %w", ds.Name, ErrNotFound)
+	default:
+		err = fmt.Errorf("HTTP status code %d: returns %s", code, raw)
 	}
-	return resp, nil
+	return resp, err
 }
 
 // DeleteDatasource deletes an existing datasource by ID.