badly-named man files cause unbuild failures

Before, if you tried to put a filename in the `man` stanza of package.json that didn't follow the rules as listed in `npm help 5 package.json`, installing would work, but upgrading or uninstalling would fail with a cryptic failure. Now, fail on build with a descriptive message, and log the same descriptive message as an error (but don't
fail) on unbuild.

Fixes #7000.