fix: production plan UX and validation message (backport #27278)

[mergify]

Changed validation message

While adding the items, the Material Request Warehouse will be use as a Target Warehouse

While downloading the raw materials if there is no conversion factor set between Purchase UOM and Stock UOM then system was throwing the below error

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This is an automatic backport of pull request #27278 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 2a8cd05 has failed:

On branch mergify/bp/version-13-hotfix/pr-27278
Your branch is up to date with 'origin/version-13-hotfix'.

You are currently cherry-picking commit 2a8cd05b44.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/manufacturing/doctype/production_plan/production_plan.json
	both modified:   erpnext/manufacturing/doctype/production_plan/production_plan.py

no changes added to commit (use "git add" and/or "git commit -a")

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