Preview: CHEM are ignored without any messages or incorrect message while we save sequence with CHEM to IDT

[NadezhdaPeskun]

Steps to Reproduce
Scenario 1

1. Switch to Macro
2. Open KET file with CHEM
   ketcher (middle Chem).zip
3. try to save to IDT
   

Scenario 2

1. Open KET file with only CHEM
   ketcher (only Chems).zip

2. try to save to IDT >> empty IDT file is created
   ketcher (Empty from only Chems).zip

Scenario 3

1. Open KET file with only CHEM
   ketcher (Chem first).zip

2. try to save to IDT >> empty IDT file is created
   

Actual behavior
CHEM are ignored without any messages or incorrect error message while Chem linked to Sugar.

Expected behavior
According to BA solution:

We should have message for CHEM as for Peptides if Chem is not in IDT library
"Cannot save molecule in IDT format - monomer(s)… doesn’t have IDT code."

as per - epam/ketcher#4495

Related:

• Macro: UI support for IDT import/export ketcher#4495
• Preview: CHEM are ignored without any messages or incorrect message while we save sequence with CHEM to IDT ketcher#4639
• Preview: In case of peptide monomers are on canvas, error "Cannot save molecule in IDT format - monomer(s) … doesn’t have IDT code." should appear. #1957
• Preview: In case of peptide monomers are on canvas, error "Cannot save molecule in IDT format - monomer(s) … doesn’t have IDT code." should appear. ketcher#4634

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