fix non-working exit code

[andrzejchm]

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This change makes sure that in case of non-zero exit code, it is properly propagated to the CLI.

Currently, no matter if there's an error or not, the flutter pub run custom_lint always exits with 0 exit code

[CLAassistant]

All committers have signed the CLA.

[rrousselGit]

Thanks! I'll look into it when I come back from holidays

[jvanduppen]

Reminder on merging this.

[rrousselGit]

Thanks for your work :)