Preserve HTTP staus codes in errors if possible

Dropbox being a REST driven service often returns useful information
in the HTTP error code.  Make sure we return these in a custom Error
type if possible.

dropbox.go

@@ -231,6 +231,31 @@ func (db *Dropbox) Auth() error {
 	return err
 }
 
+// Error - all errors generated by HTTP transactions are of this type.
+// Other error may be passed on from library functions though.
+type Error struct {
+	StatusCode int // HTTP status code
+	Text       string
+}
+
+// Error satisfy the error interface.
+func (e *Error) Error() string {
+	return e.Text
+}
+
+// newError make a new error from a string.
+func newError(StatusCode int, Text string) *Error {
+	return &Error{
+		StatusCode: StatusCode,
+		Text:       Text,
+	}
+}
+
+// newErrorf makes a new error from sprintf parameters.
+func newErrorf(StatusCode int, Text string, Parameters ...interface{}) *Error {
+	return newError(StatusCode, fmt.Sprintf(Text, Parameters...))
+}
+
 func getResponse(r *http.Response) ([]byte, error) {
 	var e requestError
 	var b []byte
@@ -245,17 +270,17 @@ func getResponse(r *http.Response) ([]byte, error) {
 	if err = json.Unmarshal(b, &e); err == nil {
 		switch v := e.Error.(type) {
 		case string:
-			return nil, fmt.Errorf("%s", v)
+			return nil, newErrorf(r.StatusCode, "%s", v)
 		case map[string]interface{}:
 			for param, reason := range v {
 				if reasonstr, ok := reason.(string); ok {
-					return nil, fmt.Errorf("%s: %s", param, reasonstr)
+					return nil, newErrorf(r.StatusCode, "%s: %s", param, reasonstr)
 				}
 			}
-			return nil, fmt.Errorf("wrong parameter")
+			return nil, newErrorf(r.StatusCode, "wrong parameter")
 		}
 	}
-	return nil, fmt.Errorf("unexpected HTTP status code %d", r.StatusCode)
+	return nil, newErrorf(r.StatusCode, "unexpected HTTP status code %d")
 }
 
 // CommitChunkedUpload ends the chunked upload by giving a name to the UploadID.
@@ -435,7 +460,7 @@ func (db *Dropbox) Thumbnails(src, format, size string) (io.ReadCloser, int64, *
 		return nil, 0, nil, os.ErrNotExist
 	case http.StatusUnsupportedMediaType:
 		response.Body.Close()
-		return nil, 0, nil, fmt.Errorf("the image located at '%s' cannot be converted to a thumbnail", src)
+		return nil, 0, nil, newErrorf(response.StatusCode, "the image located at '%s' cannot be converted to a thumbnail", src)
 	}
 	json.Unmarshal([]byte(response.Header.Get("x-dropbox-metadata")), &entry)
 	return response.Body, response.ContentLength, &entry, err