core-js: Implement postinstall error handling in a more cross-platform way

Currently the postinstall script eats errors with a || echo 'ignore' construct. This does not work in PowerShell, and therefore the package is not installable by anybody foolish or brash enough to configure their npm script-shell to be PowerShell. Surely the postinstall script itself could be made to ignore any typical errors by itself? Can somebody shed some light on why it even has that echo at the end?

I realize this may be a bit niche but it’d still be a nice little enhancement not to have to rely on specific shell features that are not universal.