Improve error message when functional method is inherited

This error can be erroneously triggered both when there are default
methods on the interface (meaning >1 methods) or when the method is
actually defined in an inherited interface (meaning 0 methods). Extend
the warning message to explain both cases.

Author: thk123

jbmc/src/java_bytecode/lambda_synthesis.cpp

@@ -121,7 +121,9 @@ static optionalt<irep_idt> interface_method_id(
       << functional_interface_tag.get_identifier()
       << " which should have exactly one abstract method but actually has "
       << implemented_interface_type.methods().size()
-      << ". Note default methods are not supported yet." << messaget::eom;
+      << ". Note default methods are not supported yet."
+      << "Also note methods declared in an inherited interface are not "
+      << "supported either." << messaget::eom;
     return {};
   }
   return implemented_interface_type.methods().at(0).get_name();