fix(grammar): Error messages for non-abstract constants

[byakuren-hijiri]

Closes #332

• I have updated CHANGELOG.md
• [ ] I have documented my contribution in Tact Docs: https://github.com/tact-lang/tact-docs/pulls/PR_NUMBER
• I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
• I have run all the tests locally and no test failure was reported
• I have run the linter, formatter and spellchecker
• I did not do unrelated and/or undiscussed refactorings

[anton-trunov]

Good catch!