rpk: improved err msg in rpk tune --output-script

[r-vasquez]

If the user used a directory it would fail and panic without giving any meaningful error.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.2.x
• v23.1.x
• v22.3.x

Release Notes

Bug Fixes

• rpk: prevent panic when using a directory instead of a filename in rpk redpanda tune --output-script

[0x5d]

LGTM logic-wise. I'll defer the review of message wording.

[r-vasquez]

Failures:

• https://buildkite.com/redpanda/redpanda/builds/37074#018a99b6-a80c-4506-af54-ddf12566f2f8/6-4276 is CI Failure (TimeoutError) in UpgradeFromPriorFeatureVersionCloudStorageTest.test_rolling_upgrade #13181
• Redpanda unit test failure: https://buildkite.com/redpanda/redpanda/builds/37074#018a9992-565a-4df4-bbc4-e52cfe83d265/6-5739 is CI Failure (critical check uploaded.size() != 0 has failed) in test_archival_service_rpfixture test_manifest_spillover test_archival_service_rpfixture.test_manifest_spillover  #13275

[vbotbuildovich]

/backport v23.2.x

[vbotbuildovich]

/backport v23.1.x

[vbotbuildovich]

/backport v22.3.x

[vbotbuildovich]

Failed to create a backport PR to v22.3.x branch. I tried:

git remote add upstream https://github.com/redpanda-data/redpanda.git
git fetch --all
git checkout -b backport-pr-13473-v22.3.x-898 remotes/upstream/v22.3.x
git cherry-pick -x 35e6b394bd53141e3257ee06535e1605a8a65b45

Workflow run logs.

[vbotbuildovich]

Failed to create a backport PR to v23.1.x branch. I tried:

git remote add upstream https://github.com/redpanda-data/redpanda.git
git fetch --all
git checkout -b backport-pr-13473-v23.1.x-858 remotes/upstream/v23.1.x
git cherry-pick -x 35e6b394bd53141e3257ee06535e1605a8a65b45

Workflow run logs.