more graceful handling of empty surfaces in surfaceFieldValue

In some workflows, surfaceFieldValue is used to obtain patch values but the patches themselves may appear or disappear during the course of the simulation. At the moment, any surface with zero faces is treated as an error (which is usually is), but this could/should be extended to emit a warning, ignore or whatever. This would allow support for these particular workflows.

The functionObjectList::errorHandlingType (currently private) provides the necessary options (default, warn, ignore, strict). These should be exposed as public and be reused for surfaceFieldValue. This would allow user-defined remedial actions. These error handling and error-codes may also be desirable for sampled surfaces etc (TDB).

cross-ref: 2243