Bugfix/#2045 Export of unresolved monomer to everything except KET and IDT should cause specific error message (but it doesn't)

[David-Kuts]

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Fix code. Add UT.

Generic request

• [ ] PR name follows the pattern #1234 – issue name
• [x] branch name does not contain '#'
• [x] base branch (master or release/xx) is correct
• [ ] PR is linked with the issue
• [ ] task status changed to "Code review"
• [ ] code follows product standards
• [ ] regression tests updated

Optional

• [ ] unit-tests written