Return the highest priority error from the descendant instead of return the very first one
The test failed because we didn't return the highest priority error (which is network error in this case).
As Manish mentioned in https://github.com/servo/servo/issues/25436#issuecomment-571065323, that's because we're using the Promise.all trick to signal loads.
If we can avoid relying on Promise.all, maybe we don't need to do a complex logic like this; instead, ideally, we should always finish the module load immediately when we hit network failure so that we don't even need to do the `max()` comparison.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#25436
- [x] There are tests for these changes
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