GH-1291 Edit log message for when function can not be located amongsr existing function registrations

Resolves #1291

Author: olegz

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/SimpleFunctionRegistry.java

@@ -302,7 +302,7 @@ private FunctionInvocationWrapper compose(Class<?> type, String functionDefiniti
 			if (function == null) {
 				if (logger.isDebugEnabled()) {
 					logger.debug("Failed to locate function '" + functionName + "' for function definition '"
-						+ functionDefinition + "'. Returning null.");
+						+ functionDefinition + " amongst existing function registrations. Will check with the bean factory");
 				}
 				return null;
 			}