"Take value from..." rules can cause warnings in factorize_secuTrial()

[PatrickRWright]

Is your feature request related to a problem? Please describe.
It is possible to run into the "Unexpected" warning in the factorization without that being a problem of the secuTrialR code. This can happen if the "Take value from..." rule is used and a value is taken which is not defined in the code mapping (cl table). Technically, that's an error in the implementation of the secuTrial database. If you run into this error its a good idea to investigate.

Describe the solution you'd like
Add this information to the warning message to give users an indication where they can look.