Use 'extensions' as field for error details

[dvic]

The June 2018 spec has reserved extensions as the field for error details. This PR updates the serializer to use this name.

From http://facebook.github.io/graphql/June2018/#sec-Errors:

GraphQL may provide an additional entry to errors with key extensions. This entry, if set, must have a map as its value. This entry is reserved for implementors to add additional information to errors however they see fit, and there are no additional restrictions on its contents.

[codecov-io]

Codecov Report

Merging #219 into master will increase coverage by 0.2%.
The diff coverage is 93.33%.

@@            Coverage Diff            @@
##           master     #219     +/-   ##
=========================================
+ Coverage   89.76%   89.97%   +0.2%     
=========================================
  Files          96       96             
  Lines       18753    17788    -965     
=========================================
- Hits        16834    16004    -830     
+ Misses       1919     1784    -135

Impacted Files	Coverage Δ
juniper/src/lib.rs	84.61% <ø> (+2.79%)	⬆️
juniper/src/integrations/serde.rs	47.5% <100%> (+7.8%)	⬆️
juniper/src/executor/mod.rs	91.52% <85.71%> (-0.17%)	⬇️
juniper_codegen/src/util.rs	77.65% <0%> (-4.54%)	⬇️
juniper/src/util.rs	96.29% <0%> (-3.71%)	⬇️
juniper/src/executor/look_ahead.rs	84.05% <0%> (-3.3%)	⬇️
...lidation/rules/overlapping_fields_can_be_merged.rs	96.55% <0%> (-1.89%)	⬇️
juniper/src/parser/document.rs	96.31% <0%> (-1.5%)	⬇️
juniper/src/value.rs	82.31% <0%> (-1.22%)	⬇️
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 56f71e9...e57d202. Read the comment docs.

[LegNeato]

Awesome, thanks for the PR! ❤️

Can you add this to the changelog and call out it is a breaking change to better align with the latest spec?

[dvic]

@LegNeato done 👍

[LegNeato]

Awesome, thanks so much! 🍻