Add logging when function can not be located

[olegz]

While FunctionCatalog will return null which is perfectly ok when function can not be found it would be more informative if it would WARN especially in cases of function composition where a single missing function will result in null lookup for the entire function definition.

[mmaask]

I don't understand this change or reason for this occurring. After upgrading Spring Cloud Stream, my entire application is full of these warnings, but code executes correctly. Function definitions are all correctly defined in spring.cloud.function.definition and all beans are created correctly. All functions also execute regardless of these warnings, why is this necessary?