FASTA: Export branched structures to FASTA should be impossible.

[NadezhdaPeskun]

Steps to Reproduce

1. Switch to Macro mode

2. Open Ket file with Peptides and branch ketcher (branch).zip

3. Save to Fasta

4. Open the saved FASTA again ketcher ((branch2).zip

Actual behavior Ketcher exports branched structures to the Fasta as two sequences.

Expected behavior As per @vanoprenko we should show error message.

Error message should appear: "Exporting branched structures to FASTA is not supported."

Issue found while testing: https://github.com/epam/Indigo/issues/1755

Versions: • OS: Windows 10 Pro (version 22H2) x64 • Browser: Сhrome • Version 122.0.6261.131 (Official Build) (64-bit)

Version 2.20.0-rc.1 Build at 2024-03-15; 15:36:45 Indigo Toolkit Version 1.19.0-rc.1.0-g8cd9f1e27-wasm32-wasm-clang-12.0.0

[AlexeyGirin]

For future verification: for the next conditions:

• If there are connections other than r2-r1, the sequence should not be exported to FASTA.
• If there are no r2-r1 connections, export multiple sequences to FASTA.
• if one sequence and additional links, that links should be ignored.