go/vt/topo: add error value to GetTablet logs

[mdlayher]

Description

We're seeing this error message a lot in logs and there is an err variable in scope, but it's not actually shown. Display it. Quote the cell name and tablet aliases for clarity as well.

Related Issue(s)

n/a, trivial cleanup.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

[vitess-bot]

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