Export isApolloError

[danielkcz]

I find it very handy for advanced error reporting to be able to recognize ApolloError. Even though it just checks for graphqlErrors property, in TypeScript world thanks to the type guard it gives additional layer.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[danielkcz]

Welp, such a tiny addition and it has increased bundle size by 21B which is over the limit 😮

[danielkcz]

@benjamn @hwillson Can you please have a look at this? It's a tiny change. I suppose the limit for bundlesize has to be increased? Cannot really do this any smaller ;)

[hwillson]

Great suggestion @FredyC, thanks!