JS API: Validate that importer result 'contents' is a string and improve ArgumentError output

[Goodwine]

See: sass/sass-spec#1834

Example output:

sass.Exception [Error]: Invalid argument (contents): must be a string but was: Buffer.: Instance of 'NativeUint8List'
  ╷
1 │ @import './foo.wxss';
  │         ^^^^^^^^^^^^
  ╵
  - 1:9  root stylesheet
  - [...]

Before using toString() for Argument error:

sass.Exception [Error]: must be a string but was: Buffer
  ╷
1 │ @import './foo.wxss';
  │         ^^^^^^^^^^^^
  ╵
  - 1:9  root stylesheet
  - [...]

Original error:

sass.Exception [Error]: NoSuchMethodError: method not found: 'charCodeAt' (receiver.charCodeAt is not a function)
  ╷
1 │ @import './foo.wxss';
  │         ^^^^^^^^^^^^
  ╵
  - 1:9  root stylesheet
  - [...]

Fixes #1550

[nex3]

Can you add a test for this to sass-spec as well?

[Goodwine]

Can you add a test for this to sass-spec as well?

I wasn't sure where were the tests actually, but I found them now 🙂

Added tests and updated to also handle legacy importers which I realized just now via testing that it wasn't handled (yay tests)