BadSharedToken: make AEPST err msg optional

Add a configuration entry (aaf.av.alert_on_sharedtoken_errors - default
true) to tweak whether validation errors for sharedToken trigger the
BadSharedToken error page - obscuring the actual attribute list.
Messages: update messages-base from applicationbase
Messages: fix typos in error messages
Tests: SnapshotControllerSpec: turn BadSharedToken on

Adjust SnapshotControllerSpec to set
config.aaf.av.alert_on_sharedtoken_errors = true

to properly test SnapshotController even with changes introduced in
131c033f134670a3bcc6a9bf8a4f44a4863f3b4b
Version: internal Tuakiri release 1.0-tuakiri1