[Manure] Add a more descriptive error message when a parlor manure stream is assigned to a incorrect processor type

[elle-andreen]

In the manure module, the manure stream representing the milking parlor manure must be directed to a ParlorCleaning handler in the manure module. This is done through assignment of the desired parlor cleaning handler as the first_parlor_processor for the parlor manure stream (in the animal input file). If any other type of processor is assigned as the first parlor processor, a NoneType error is returned. This processor type selection error should be caught during the data validation step (maybe via cross-validation?) and a more descriptive error message should be returned, e.g., "Milking parlor manure stream from (pen #) assigned to a processor with type (type). Milking parlor manure streams can only be assigned to ParlorCleaning manure processors." This will help more immediately clarify where the user made an error in setting up their inputs.