Feat/return errors

[syedhasandigi]

Pull Request

Checklist

• Have you read Digital Catapult's Code of Conduct?
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.

PR Type

Please delete options that are irrelevant.

• Feature

Linked tickets

https://digicatapult.atlassian.net/browse/NIDT-123

High level description

dtdlParser now returns an error object instead of null for a failed parse

Detailed description

Function dtdlParser will return a ModelingException instead of null, type is now
DtdlObjectModel | ModelingException

Operational impact

Consumption in the dtdl-visualsation-tool where null is handled must be updated to handle ModelingException