fix displaying of graphql resolver errors

[pieh]

Currently if something in graphql resolver will throw error we will see somethink like this in console:

run graphql queriesError running queryRunner TypeError: Cannot read property 'componentPath' of undefined
    at module.exports (/Users/mike/dev/gatsby-v2/examples/using-contentful/node_modules/gatsby/dist/internal-plugins/query-runner/query-runner.js:47:36)

This fixes this and displays formatted error message

[gatsbybot]

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[m-allanson]

👍 looks good!

Is it worth writing a couple of tests for formatErrorDetails? As a way of documenting expected inputs and outputs.

[pieh]

Is it worth writing a couple of tests for formatErrorDetails? As a way of documenting expected inputs and outputs.

Defenitely won't hurt :) I'll add those

[m-allanson]

Nice 👍