Error Message Improvements

[cknitt]

From the retreat notes:

• Subtyping (relevant information is not currently tracked in the compiler)
• Interface mismatches (harder to read with JSX4)
• Get rid of OCaml syntax in error messages
• Code Actions for fixing error messages / refactoring

Also:

• Function application (incorrect number/types of arguments) Improve error messages for function calls with mismatching args #7055

[nojaf]

Hi @zth, any chance you could add some pointers on how the Code Actions for fixing error messages / refactoring part would play out?