Upgrade/Install: Better error message when uploading theme as plugin and vice versa.

[karthikeya-io]

Trac ticket: https://core.trac.wordpress.org/ticket/28845

The PR updates error messages to indicate if a theme is uploaded as a plugin and vice versa.

Uploading Plugin in place of theme

Before	After

Uploading Theme in place of plugin

Before	After

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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Props karthikeya01.

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