fix: avoid possible infinite loop by accessing observables in error handler (fix vuex#1505)

[ktsn]

fix vuejs/vuex#1505

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
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• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

The original issue is vuejs/vuex#1505
When we touch some observables in errorCaptured hook, it is registered as dependency of the rendered component. Then the infinite rendering loop happens if we update the value inside the hook.

Updated reproduction which does not have Vuex store is here. https://codesandbox.io/s/ypvy7zn9xx