cmd/age: print a better error message when plugins are missing

[FiloSottile]

If we choose to print the first argument of unrecognized stanzas, we should update the plugin spec to mention authors should put something recognizable and not sensitive in there, as in theory it's not special.

See #443 for a use case.

[FiloSottile]

Alternatively, we could define a stanza type specifically for user hints. Plugins could produce

-> hint
download plugin from example.com

based on which we could print

age: hint: the file includes a suggestion: download plugin from example.com

These stanzas would not count for exclusive types like scrypt.

[AnomalRoil]

I think the hint approach seems like a better option, but means "bulkier" stanzas. Not sure "how cheap" we consider storage?