fix(#3851): Improve error message for invalid program definition

[volodya-lombrozo]

This pull request improves the error messages in the EO parser by providing more informative and clear descriptions when a program declaration is invalid. The updated messages now specify that a valid program definition is expected, consisting of one or more meta declarations followed by an object list, but an unexpected construct was encountered.

Closes #3851.

[github-actions]

🚀 Performance Analysis

Test	Base Score	PR Score	Change	% Change	Unit	Mode
benchmarks.XmirBench.xmirToEO	157.353	148.694	-8.659	-5.50%	ms/op	Average Time
benchmarks.XmirBench.xmirToPhi	177.725	164.581	-13.145	-7.40%	ms/op	Average Time
benchmarks.XmirBench.xmirToSaltyPhi	154.587	142.680	-11.908	-7.70%	ms/op	Average Time

✅ Performance gain: benchmarks.XmirBench.xmirToEO is faster by 8.659 ms/op (5.50%)
✅ Performance gain: benchmarks.XmirBench.xmirToPhi is faster by 13.145 ms/op (7.40%)
✅ Performance gain: benchmarks.XmirBench.xmirToSaltyPhi is faster by 11.908 ms/op (7.70%)

[volodya-lombrozo]

@maxonfjvipon What do you think about this error message?

[volodya-lombrozo]

The #4026 PR has already solved the main part of the #3851 issue. Here we just change the error message and close the puzzle.

[volodya-lombrozo]

@yegor256 Could you merge these changes, please?