go-to-protobuf: small fixes to improve debuggability

[nicks]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Small fixes to make the go-to-protobuf tool easier to debug when it's failing:

• If a package isn't found, print the package name instead of segfaulting
• Init klog flags, like in other code-generator/cmd packages, to turn on verbose logging

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[nicks]

/assign @smarterclayton

[fedebongio]

Clayton do you need help on this one? if so please untriage. Thanks!
/triage accepted

[ash2k]

/ok-to-test
/lgtm

[wojtek-t]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nicks, wojtek-t

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Needs approval from an approver in each of these files:

• staging/src/k8s.io/code-generator/cmd/go-to-protobuf/OWNERS [wojtek-t]

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[fejta-bot]

/retest
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[nicks]

thanks @ash2k @wojtek-t !