https://chromium-review.googlesource.com/c/chromium/src/+/1560533
https://chromedevtools.github.io/devtools-protocol/tot/Page#method-getInstallabilityErrors

It tests against the real implementation, so we won't have a gap.
https://cs.chromium.org/chromium/src/chrome/browser/installable/installable_logging.cc?l=15-57

This also means we can probably drop manifest parser, manifest-values, etc.
Their offline check is a little different though.

The strings returned are hardcoded english.. but then again so are our current strings. ;)