fix(gencli): dont return iterator.Done

[liveFreeOrCode]

When generating a Paged method: https://google.aip.dev/132, the err at the end will always be an iterator.Done (as long as the iterator was built successfully).

List CLI methods should not return iterator.Done method.

[noahdietz]

I keep finding that folks actually use this! haha Thanks for the fix, you might need to update the baselines as well.

[noahdietz]

you might need to update the baselines as well.

or not! that's a gap...would you mind adding a test case to the cmd_file_test.go? Then run
go test github.com/googleapis/gapic-generator-go/internal/gencli -update_golden to write the baseline to disk assuming it is correct

[quartzmo]

@liveFreeOrCode Thank you for this contribution! 🎉