Monomorphisation: add a better error message

Motivation and Context

Monomorphisation cannot treat non-variable instantiations (which was guaranteed by previous typer).

Description

This MR adds a better message for monomorphisation error until we come up with a better strategy for monomorphisation.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Before change

$ cat monomorphisation_fail.mligo 
let f (_ : unit) s = ([], s)

let main ((p, s) : unit * unit) : operation list * unit = f p s
$ ligo compile contract monomorphisation_fail.mligo 
Internal error: Monomorphisation: cannot resolve non-variables with instantiations

After change

$ cat monomorphisation_fail.mligo 
let f (_ : unit) s = ([], s)

let main ((p, s) : unit * unit) : operation list * unit = f p s
$ ligo compile contract monomorphisation_fail.mligo 
File "monomorphisation_fail.mligo", line 3, characters 58-63:
  2 | 
  3 | let main ((p, s) : unit * unit) : operation list * unit = f p s

Cannot monomorphise the expression.

Checklist:

• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).