Handle errors when loading `next.config.js`

[ehmicky]

There are many reasons why loading next.config.js might fail such as:

• Invalid configuration value (example)
• Syntax error (example)
• General error in the JavaScript logic, throwing an exception (example)

Those errors are currently reported as plugin bugs. We should report those as user errors instead by adding a try/catch + utils.build.failBuild() block.

[ehmicky]

Other example here:

Specified i18n.defaultLocale should be included in i18n.locales.
See more info here: https://err.sh/next.js/invalid-i18n-config 

[ehmicky]

This bug was fixed in #99 for:

https://github.com/netlify/netlify-plugin-nextjs/blob/61b61f57d58ae095e3522c86013c75823911179b/helpers/hasCorrectNextConfig.js#L17

However, the configuration is loaded also here:

https://github.com/netlify/netlify-plugin-nextjs/blob/61b61f57d58ae095e3522c86013c75823911179b/src/lib/helpers/getNextConfig.js#L11

We should use a similar try/catch/failBuild block there as well.

Bugsnag errors: one, two

[ehmicky]

First part at #114

[ehmicky]

Fixed by #124.