Handling of core.composeEmail errors

[ivan-ushakov]

close #411

---

Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)

---

To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[ivan-ushakov]

Maybe ComposeMessageError should be updated to provide more human readable error description.