PGP missing keyring file test fails on Windows

jameswing reported on the mailing list that `TestEncryptContent.testShouldValidatePGPPublicKeyringExists` was failing on a Windows 10 machine but passing on a Linux machine. The cause of this is that the test was inspecting the validation result and expecting a specific message which was provided on Mac OS X and Linux systems but differs on Windows.

The resolution is to inspect the validation result for the fully-qualified exception class name "java.io.FileNotFoundException" rather than OS-dependent messaging.