Change mutation errors field from [Error] to [Error!]

[fowczarek]

I want to merge this change because resolve #2746

Screenshots

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #3489 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3489   +/-   ##
=======================================
  Coverage   89.87%   89.87%           
=======================================
  Files         241      241           
  Lines       13103    13103           
  Branches     1322     1322           
=======================================
  Hits        11776    11776           
  Misses        922      922           
  Partials      405      405

Impacted Files	Coverage Δ
saleor/graphql/core/mutations.py	92.68% <ø> (ø)	⬆️

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[codecov]

Codecov Report

Merging #3489 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3489   +/-   ##
=======================================
  Coverage   89.87%   89.87%           
=======================================
  Files         241      241           
  Lines       13103    13103           
  Branches     1322     1322           
=======================================
  Hits        11776    11776           
  Misses        922      922           
  Partials      405      405

Impacted Files	Coverage Δ
saleor/graphql/core/mutations.py	92.68% <ø> (ø)	⬆️

---

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 a1bb298...f7bb3c8. Read the comment docs.

[maarcingebala]

@dominik-zeglen How much does it affect Dashboard 2.0 code? Can we merge it or is your intervention required here?

[dominik-zeglen]

I think we can get rid of (now) pointless checks if(data.something.errors === null "" data.something.errors.length === 0). Do it now?