ref(angular): Extract zonejs error unwrapper into a dedicated function

[theofidry]

I believe extracting this into a dedicated function helps to:

• Convey the intent: instead of having a statement and a comment explaining the statement, you directory have a descriptive and explanatory function name.
• This also to further encapsulate additional information, here about a possible refactoring that would make it clearer.
• Bonus: this was the only reason why let was being used, so now const can be used

[AbhiPrasad]

Hey @theofidry thanks for opening PRs! I've taken the liberty to update your PR title based on our contributing docs. We'll review your contribution asap - please let us know if you have any questions.

[theofidry]

@AbhiPrasad thanks!