GH-1265 Add logging for failed function lookup

Resolves #1265

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

@@ -300,6 +300,8 @@ public class SimpleFunctionRegistry implements FunctionRegistry {
 		for (String functionName : functionNames) {
 			FunctionInvocationWrapper function = this.findFunctionInFunctionRegistrations(functionName);
 			if (function == null) {
+				logger.warn("Failed to locate function '" + functionName + "' for function definition '"
+						+ functionDefinition + "'. Returning null.");
 				return null;
 			}
 			else {