Invalid imports should not crash the preview server

[mbostock]

This causes the preview server to render an error on load instead of just an invalid import:

import "npm:@observablehq/does-not-exist";

And if the code block is added to an existing page, the preview server crashes!

npm:@observablehq/does-not-exist → …/framework/src/npm.ts:224
    if (!response.ok) throw new Error(`unable to fetch: ${href}`);
                            ^

Error: unable to fetch: https://data.jsdelivr.com/v1/packages/npm/@observablehq/does-not-exist/resolved
    at <anonymous> (…/framework/src/npm.ts:224:29)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

We should catch such an error and let the import fail on the client instead.

(It’s desirable to crash the build, though, so you don’t accidentally deploy a broken site.)