Patch failures should cause composer install to fail

[danepowell]

Currently, if a patch cannot be applied during composer install, it spits out a warning and then merrily continues on its way. This makes the patching process very fragile, as our automated tests won't catch failed patches.

There's an upstream issue to address this, but it's not going to be fixed in the current branch of composer-patches: cweagans/composer-patches#40

We can also work around it manually by tweaking our test scripts, which is the better approach in the interim:
cweagans/composer-patches#31
cweagans/composer-patches#46

However we fix it, we should submit an identical solution to Lightning.