go/vt/vtctl: fix nilness issues and error scopes

[mdlayher]

Description

Fixes:

✔ ~/src/github.com/vitessio/vitess/go/vt/vtctl [main|✔] 
16:16 $ nilness ./...
/home/matt/src/github.com/vitessio/vitess/go/vt/vtctl/workflow/server.go:1445:14: tautological condition: non-nil != nil
/home/matt/src/github.com/vitessio/vitess/go/vt/vtctl/workflow/traffic_switcher.go:383:10: impossible condition: nil != nil
/home/matt/src/github.com/vitessio/vitess/go/vt/vtctl/grpcvtctldserver/server.go:5011:35: tautological condition: non-nil != nil

In particular the vtctld server had some confusing error scopes so I tried to infer the correct errors to display based on the surrounding context. It would appear the error message under conn.ListDir should show that error, so the existing err variable in scope there would have been completely unrelated to the error being checked.

Related Issue(s)

Updates #14684.

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

n/a

[vitess-bot]

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[frouioui]

oof, this is a good catch, the previous line 5000 was actually wrong