core: log unit failure with type-specific result code

This slightly changes how we log about failures. Previously,
service_enter_dead() would log that a service unit failed along with its
result code, and unit_notify() would do this again but without the
result code. For other unit types only the latter would take effect.

This cleans this up: we keep the message in unit_notify() only for debug
purposes, and add type-specific log lines to all our unit types that can
fail, and always place them before unit_notify() is invoked.

Or in other words: the duplicate log message for service units is
removed, and all other unit types get a more useful line with the
precise result code.

src/core/path.c

@@ -457,6 +457,9 @@ static void path_enter_dead(Path *p, PathResult f) {
         if (p->result == PATH_SUCCESS)
                 p->result = f;
 
+        if (p->result != PATH_SUCCESS)
+                log_unit_warning(UNIT(p), "Failed with result '%s'.", path_result_to_string(p->result));
+
         path_set_state(p, p->result != PATH_SUCCESS ? PATH_FAILED : PATH_DEAD);
 }