added errorRedirect to middleware opts

[thebells1111]

errorRedirect is an object that can be passed to middleware that allows server side redirect on an error code.

errorRedirect: {errorStatus: 404, statusCode: 301, location: '/'}

The above object can be passed to allow a server side redirect to the specified location based on the errorStatus.
errorStatus can be a number, string, or array of number/string, and will default to 404
statusCode can be a number/string for the reroute status code; i.e. 301, 302, and will default to 301
location will be a string with the path to redirect to, and will default to '/'

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Tests

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[benmccann]

Thanks for putting together this PR! Sapper never had a good method of handling configuration, which made it difficult to know how to integrate this feature. However, that's been addressed in SvelteKit by adding a svelte.config.cjs file and adapters. I'm going to go ahead and close this PR since we won't be adding the feature to Sapper. You might like to check out SvelteKit instead