Add an Error Boundary for the results page, and add more tests around the react router behavior

[julienw]

Please look only at the last commits that are not prefixed with a mention.

The 2 first commits are preparatory to the Error Boundary component. They make it easier to have a similar visual design. Commit 3 adds a simple error boundary and configures it with the router. Commit 4 adds new tests for the router and the error boundary.

Most of the remaining missing coverage will be covered when I'll work on fetching the recent revisions using the react router, that's why I didn't cover them here.

You can test the error boundary by requesting with wrong parameters, for example:

• no parameters => https://deploy-preview-600--mozilla-perfcompare.netlify.app/compare-results
• no baseRepo => https://deploy-preview-600--mozilla-perfcompare.netlify.app/compare-results?baseRev=foo
• not the same amount of newRevs and newRepo => https://deploy-preview-600--mozilla-perfcompare.netlify.app/compare-results?baseRev=foo&baseRepo=mozilla-central&newRev=spam&newRepo=try&newRepo=mozilla-central

[netlify[bot]]

✅ Deploy Preview for mozilla-perfcompare ready!

Name	Link
🔨 Latest commit	43f6dc93354d059f8174774006549c7047afc347
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a552110) 92.54% compared to head (43f6dc9) 93.63%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## beta #600 +/- ## ========================================== + Coverage 92.54% 93.63% +1.09% ========================================== Files 62 64 +2 Lines 1395 1414 +19 Branches 217 219 +2 ========================================== + Hits 1291 1324 +33 + Misses 100 88 -12 + Partials 4 2 -2 ```

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