tweaked error messages
error messages - more context, remove grammar rule info from message seen by end user
reworking error messages
more error message formatting and tweaks
yet another error msg format tweak
updated error message expected by unit test
Merge branch 'develop' into feature/issue-623-parser-error-msg
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-623-parser-error-msg
Merge branch 'feature/issue-623-parser-error-msg' of https://github.com/stan-dev/stan into feature/issue-623-parser-error-msg
Merge pull request #1083 from stan-dev/feature/issue-623-parser-error-msg

Fixes #623. Feature/issue 623 parser error msg