Inappropriate error message displayed when importing an invalid string

Found while testing #3541

**Steps to Reproduce:**

1. Open Macromolecules - Flex mode (clean canvas)
2. Paste the following invalid BILN string from the clipboard: `{}`
3. Click `Open as New` or `Add to Canvas` button

**Actual Result:**
An inappropriate error message is displayed

**Expected Result:**
"The string cannot be interpreted as a valid BILN string." error message is displayed

As per Req:

As per BA:

**Found in Environment:**

- Ketcher Version 3.17.0-rc.1 Build at 2026-06-02; 21:27:10
- Indigo Version 1.45.0-rc.2.0-g168fb85f2-wasm32-wasm-clang-23.0.0
- Chrome Version 149.0.7827.156 (Official Build) (64-bit)
- Win11

Reproduce the issue in Macromolecules Flex mode by pasting `{}` and using Open as New or Add to Canvas. Trace the invalid BILN import entry point and the displayed error handling; done means the exact message "The string cannot be interpreted as a valid BILN string." appears for this input.

Written by the indexing model from the issue text.