Fix yarn behaviour when scripts are failing

[sth]

Summary
Fix for #5451, #5457.

The test case in the first commit fails for pre-node5 builds, in the way described in the links issues. The problem are classes deriving from Error, which are problematic in ES5 (babel). Adding babel-plugin-transform-builtin-extend fixes the issue in this case.

Why now?
The error surfaced because the current yarn build published on npm is a pre-node5 build while for example v1.3.1 seems to have been created by the more modern build configuration. This can be seen by checking if code like "class A extends B" got compiled to ES5 or not.

[buildsize]

This change will increase the build size from 10.51 MB to 10.52 MB, an increase of 9.31 KB (0%)

File name	Previous Size	New Size	Change
yarn-[version].noarch.rpm	910.69 KB	911.87 KB	1.18 KB (0%)
yarn-[version].js	3.96 MB	3.96 MB	41 bytes (0%)
yarn-legacy-[version].js	4.11 MB	4.11 MB	6.19 KB (0%)
yarn-v[version].tar.gz	916.08 KB	917.1 KB	1.03 KB (0%)
yarn_[version]all.deb	676.31 KB	677.19 KB	900 bytes (0%)

[arcanis]

The error surfaced because the current yarn build published on npm is a pre-node5 build while for example v1.3.1 seems to have been created by the more modern build configuration.

That looks weird :/ cc @BYK @Daniel15 did we change something recently?

[rally25rs]

wow, nice find. 👏 I never would have guessed that err instanceof MessageError wouldn't work. I'm curious what happened in the build process, but either way, it looks like this babel plugin fixes it.

[Daniel15]

It looks like we haven't changed the npm publishing code in quite a while: https://github.com/yarnpkg/yarn/blame/master/scripts/update-npm.sh. Maybe the build config changed, but I'm not sure.