Small syntax error improvements

[WardBrian]

Three small things:

1. Make an error message about bad declarations more specific.
2. Add some more example models that have 'improvable' errors (it's difficult to improve with the current menhir tech, but there are other options that could do)
3. Improve the error reporting when the error is at the end of the file. Currently, the ^ points at the line before, which can be confusing.

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Improved syntax errors in some cases.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (bcc346b) to head (189890e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1580   +/-   ##
=======================================
  Coverage   90.30%   90.30%           
=======================================
  Files          65       65           
  Lines       10043    10046    +3     
=======================================
+ Hits         9069     9072    +3     
  Misses        974      974           

Files with missing lines	Coverage Δ
src/frontend/Parse.ml	88.57% <100.00%> (ø)
src/middle/Location.ml	95.91% <100.00%> (+0.26%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.