Add processing mode to expansion

[filip26]

Is your feature request related to a problem? Please describe.
Undefined entries are silently omitted during expansion.

Describe the solution you'd like
Add a new configuration option to expansion allowing to set how the processor should proceed when an undefined term that cannot be expanded is found.

enum Mode  {  
   Ignore,   // ignore, the current and default behavior
   Fail,     // stop processing with an error
   Warn      // print warning to log
}