feat(engine+ui): Improve error messages [2/N]

[daryllimyt]

Makes progress on #307

Features

• Add a "locator" of the form [ACTIONS.<action.ref> -> <location>] to error and validation messages. e.g. [ACTIONS. my_action -> run_if] <some error about run_if statement>
• Clean up parser error messages
  • Avoid showing too low-level information like allowed terminals

Changes

See commit messages

New screens

Old Screens

• The locator notation is outdated in the following screenshots