Partially fix errors when using DummyKeyringModule

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.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188342973

java/google/registry/keyring/api/pgp-public-keyring.asc

@@ -1,20 +1,19 @@
 -----BEGIN PGP PUBLIC KEY BLOCK-----
-Version: GnuPG v1
 
-mI0EV88FqQEEAJMqfxk9spXJp/FU6cWH2f1dvQMgDamHyhPDynk5smnNn9vH8HF4
-gyzkqOJm7rKv5JACQNuiQPCUVNUhURK+TFg94dWf2+dbTVPieVDb/h7MCqM24aXs
-HrUqXnQkymuBmj5W9tGHNhZhxlyVkM071rHKmFa4gVqsyB0OXxMLQi4rABEBAAG0
-OERvbWFpbiBSZWdpc3RyeSA8ZG9tYWluLXJlZ2lzdHJ5LXVzZXJzQGdvb2dsZWdy
-b3Vwcy5jb20+iLgEEwECACIFAlfPBakCGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4B
-AheAAAoJEIhf2KA0zyn/qVkD/Ai6QRPuI5Wn+6bh94shougFomVZ1vp1Ec+BAplo
-V82DkJoAsJ5U7CwEo0DpgIIK1pJrWXpsjPOILwuKLfsMHmE0X0z0DfnSuBQUe0rN
-7kEAAO0+SkOljnSc+6DCRA/tQSMIxymLHR8Nflwn9sn29rvDl/Dctf8a4G4sd6az
-AYG6uI0EV88FqQEEAMbkPdBVfS0c5832XuPyFBhkNS/5LvcbgNUxnk57qkUfZHU6
-H2cOadmtCofdecehJ1S6ILAL7IcCIg2Wq2iUYlywaxex3+HyoHNIBzbZZFUgnqMN
-ZBknA1zqUNNXXgxX1GI0KvOl0n8AqIJ8Ozuet+pDbEu03tI3z2Zm4CXZXRlzABEB
-AAGInwQYAQIACQUCV88FqQIbDAAKCRCIX9igNM8p/3GXA/9GueNC3Q98b/SaxMEt
-mzD2FCrcKsP3crIi55L1tYUAY+2yVIUKyC296faj3LmyjzzgzT31MSjo0F+80zE3
-eBe8FRl6AEa6oqr5gqyvVy9VJ6mfqH77yHdknKealD9IguZHPKHd0+uAxDyT6jLN
-vxq43zBsEU8xGyHadtiYaNBLMg==
-=v+Qg
+mI0EWjxpEgEEALwwDbFBdi8Rt4tXWIyjgYLeVEVi+tQJqt6eHCha58Jgfp0aJ6f8
+eGIHR6IMXrvYngy254dDYztRoYmDEChBMb1zo73S+WYUWE9n3rtTUxN1j80u8Mpt
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+KVRlc3QgUmVnaXN0cnkgPHRlc3QtcmVnaXN0cnlAZXhhbXBsZS5jb20+iLgEEwEC
+ACIFAlo8aRICGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJELyDn6y24BDd
+jXoD/iVfouA8T079430Q3KRsvMgwDYiKARUsgEaajKoNPk31f1MqbsoxUxn5y7kL
+/qLHHjirT3rx/IAnO+/fjqrasjQ3D6KExuSvnlGEe+z2LX87CpP2f997ULRCQ2/F
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+lCN8MK8exDQQvPZwYVQOuoP3cHriCslLGznB943URcuxXz6R7F7WixYUeVVpQ4J0
++gFubR8PfThDCtHQyP+uYx9U+EVWIvuIZIchdjl9ABEBAAGInwQYAQIACQUCWjxp
+EgIbDAAKCRC8g5+stuAQ3XjWA/0YBNgKiKGbmKBtnQUvxg89vla8elMQDhaB0ghR
+QJDRNgnZk4J0rSLeptvXFFEEmrTTqpKITi+pDt8c7wfGYRZo8uWw+95sabf09S7N
+XJbHwjbD7T+pzG3kvZVXNtkWnr5ihr6Z5Fiw7vuADNS+pQbXItajowPilJQ1gkDz
+rsEH+Q==
+=ilBa
 -----END PGP PUBLIC KEY BLOCK-----