expose ERROR symbol from error-handler

[betarabbit]

Summary

(If you have not already please refer to the contributing guideline as described
here)

• Tell us about the problem your pull request is solving.
  Per Error handling paragraph in README, ERROR symbol should accessible by const { ERROR } = require('feathers-knex'), however it is not exposed as expected.

• Are there any open issues that are related to this?
  No.

• Is this PR dependent on PRs in other repos?
  Yes

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[daffl]

Thanks! I thought it was even tested but apparently error[undefined] works just fine.