Improve SubprocessException

[srawlins]

I was getting errors during a local flutter update-packages call, and SubprocessException was not really any help. This added information helps.

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[kevmoo]

considered extending https://api.dart.dev/stable/3.3.0/dart-io/ProcessException-class.html ?

[srawlins]

Did not :D but I guess we could shave a few lines.

[kevmoo]

For all of those folks who want to catch...uh...

You know, not a biggy. Just pondering...