client: don't hide context errors

Instead of reformatting error from the request action, we wrap it,
allowing the cause to be recovered. This is important for consumers that
need to be able to detect context errors, such as `Cancelled` and
`DeadlineExceeded`.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

request.go

@@ -14,6 +14,7 @@ import (
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/versions"
 	"github.com/docker/docker/client/transport/cancellable"
+	"github.com/pkg/errors"
 	"golang.org/x/net/context"
 )
 
@@ -131,7 +132,8 @@ func (cli *Client) sendClientRequest(ctx context.Context, method, path string, q
 				}
 			}
 		}
-		return serverResp, fmt.Errorf("An error occurred trying to connect: %v", err)
+
+		return serverResp, errors.Wrap(err, "error during connect")
 	}
 
 	if resp != nil {