vendor: add sourcegraph/s3cache and dependencies

Adds:
 - github.com/kr/http/transport
 - github.com/sqs/s3
 - github.com/sqs/s3/s3util
 - sourcegraph.com/sourcegraph/s3cache

vendor/github.com/sqs/s3/s3util/error.go

@@ -0,0 +1,29 @@
+package s3util
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+type respError struct {
+	r *http.Response
+	b bytes.Buffer
+}
+
+func newRespError(r *http.Response) *respError {
+	e := new(respError)
+	e.r = r
+	io.Copy(&e.b, r.Body)
+	r.Body.Close()
+	return e
+}
+
+func (e *respError) Error() string {
+	return fmt.Sprintf(
+		"unwanted http status %d: %q",
+		e.r.StatusCode,
+		e.b.String(),
+	)
+}