raise ArgumentError on unknown directive for Array pack / String unpack

[Th3-M4jor]

Makes it so that Array#pack and String#unpack will raise an ArgumentError on unknown directives.

Since it appears that ruby/spec does not have any specs for unknown directives on String#unpack, I opened a PR there to add one.

[andrykonchin]

Looks good to me.

I suppose the change fixes both #pack and #unpack methods. Could you please add changes of the ruby/spec#1218 PR into this one? ruby/spec's tests in TruffleRuby are being synced periodically with the main ruby/spec Git repo.

Also it makes sense to specify exception message (unknown pack directive ...) in the new test and probably in the existing one.

[Th3-M4jor]

Had to make a slight change to the SimplePackListener interface so that the error messages would more closely match those from MRI as it differentiates between pack/unpack in the error message.

[andrykonchin]

Thank you!