Fix: improve monomorphisation error message

Motivation and Context

Related Issue: #1516 (closed)

Description

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

Example file

$ cat annotate_arrow.mligo 
let f (_:unit) (_:nat option) = None

After fix

$ ligo compile expression cameligo "f" --init-file annotate_arrow.mligo
File "annotate_arrow.mligo", line 1, characters 0-36:
  1 | let f (_:unit) (_:nat option) = None

Cannot monomorphise the expression.
The inferred type was "unit -> ∀ a . option (nat) -> option (a)".
Hint: Try adding additional annotations.

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).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).