chg: ignore NoSuchFileException for Pwned Passwords

If we can’t open the Pwned Passwords database because the file does not
exist, there’s no need to print a detailed stack trace: the warning
message should be enough to gently inform the system administrator that
they can improve their security by installing the database. Any other
errors (e. g. permission errors) are still reported.

This is mainly motivated by the dozens of NoSuchFileException stack
traces in CI builds, which this commit should silence.

Change-Id: Id08afc1600a70acfc49b2c4335b533949413b09a