This fixes a few problems encountered when building and running according to the Install Guide using the DummyKeyring. It's still failing when trying to parse the JSON credential, which I haven't solved, but before proceeding I wanted to get agreement that it needs to be fixed at all since the best we could do is provide a valid format (as with the PGP keyrings), but the metrics logging will still fail for a different reason (i.e. the credential will not work for the GC project).
Things fixed in this PR:
Fix format string causing MissingFormatArgumentException in FrontendServlet
when keyring fails to load.
Include exception cause in VerifyException in PgpHelper.
Replace dummy PGP keyrings with ones without a password, as code expects.
Document how the PGP keyrings are created.
P.S. I see a tab character snuck into PgpHelper. I'll fix that if you're interested in this PR.
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c4f1be4baa