feat: better error message for preview if build output doesn't exist

[gtm-nayan]

closes #10334

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[ghostdevv]

Suggested change

	throw new Error(`Server files not found at ${dir}, did you run \`build\` first?`);
	throw new Error(`Server files not found at ${dir}, did you run \`vite build\` first?`);

[gtm-nayan]

I had that in my first iteration but removed the vite part because people usually run the build script in their package.json instead of invoking the vite CLI by itself.