fix(ssr): reject serverPrefetch with error

[dsanders11]

Currently on error the promises used in serverPrefetch are being rejected with no value. Vue doesn't consider this to be an error, which leads to a broken response where the component is not rendered, but SSR considered the render successful, so it's not possible to detect the error.

This should fix #897 and #900.

@Akryum, a quick merge would be appreciated, as currently it's a bit worrisome to use in production since a transient network error could lead to clients getting a broken response.

[vsorrokin]

Current version of vue-apollo can't be used in production SSR app, this is critical. Please merge this PR ASAP.

[kepek]

👍

[dargmuesli]

Hmm, releasing this as a patch-level change made my project's build break. Something that should be a major-level change.

What's the correct way to handle this error? I guess I could turn off SSR using ssrMode: false within new ApolloClient()?

[brunomperes]

I think this is a bit arguable, for example one could perform a retry on the client which will automatically happen if the data is not on the exported cache.

With the approach this PR added, there's no way to retry it. The whole server rendering will fail if any request fails. In comparison, using REST one could catch the error and update the markup without causing the whole SSR to fail. Now there is no option.

Since there seem to have use cases, should we instead have an option for failing on SSR requests? Opened #1046 to discuss this.