fix: ArgResults.operator[] should throw Exception not Error

[eseidel]

ArgResults.operator[] and ArgResults.option() would throw ArgumentError instead of Exception when the user would fail to pass all expected arguments.

Errors are only supposed to be thrown on programmer error so changed to throw ArgParserException instead.

Fixes #872

This is a breaking change!

[eseidel]

I'm honestly not sure if this is worth landing, as there is a non-trivial risk of breaking lots of apps in the wild.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
args	None	2.7.0	2.7.0	2.7.0	✔️

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Coverage ✔️

File	Coverage
pkgs/args/lib/src/arg_results.dart	💚 88 %

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