The structure with Rmember saved in CML-format cannot be open.

**Steps to reproduce**

1. Create any structure, for example, chain.
2. With RGroup Fragment tool assign this structure as RGroup member, for example, R3.
3. Click "Save as..." button.
4. Select CML format. Click "Save to file".
5. Clear the canvas.
6. Open the file saved at 4th step.

**Actual result**
When user saves the R Member structure no error or warning message appears at the screen. But when user tries to open this saved file, error message "Syntax error: unexpected token < in JSON at position 0" appears, the file isn't opened.

**Expected result**
I suppose that there are two variants to solve this problem:
1) We should support such structure and Ketcher should open them in correct way.
2) If the first variant is impossible, we should not to allow user to save such structures, warn user with message. The possibility of saving the files that cannot be opened should be disabled.