feat: raise pr with warning if lock file generation fails (#994)

Previously, upgrades with failing lock files were not raised as PRs. Usually this is because of missing private module configuration. Now, Renovate will raise a PR but add a comment warning of the error, with the error log. It's raised as a comment because we now won't need to regenerate the lock file error every run, and we don't want to "lose" it if the PR description gets updated.

Closes #600