os/session_policy: warn if no policy exists

The situation where a 'Session_policy' is constructed for a label with
no matching policy is in almost all cases a configuration problem.
A diagnostic message eases pin-pointing such mistaks. By adding the
message to the 'Session_policy', servers don't need to manually handle
the exception to provide diagnostic information. This simplifies the
server code in many components.

repos/gems/recipes/raw/motif_wm/decorator_init.config

@@ -12,6 +12,7 @@
 		<resource name="RAM" quantum="8M"/>
 		<config>
 			<controls> <maximizer/> <title/> </controls>
+			<default-policy/>
 		</config>
 		<route>
 			<service name="ROM"    label="window_layout">     <parent label="window_layout"/>     </service>

repos/gems/recipes/raw/motif_wm/wm.config

@@ -45,6 +45,7 @@
 		<config>
 			<policy label_prefix="decorator" role="decorator"/>
 			<policy label_prefix="layouter"  role="layouter"/>
+			<default-policy/>
 		</config>
 		<route>
 			<service name="ROM" label="resize_request"> <child name="report_rom"/> </service>

repos/gems/recipes/raw/themed_wm/decorator_init.config

@@ -14,6 +14,7 @@
 		<config>
 			<libc/>
 			<vfs> <tar name="plain_decorator_theme.tar"/> </vfs>
+			<default-policy/>
 		</config>
 		<route>
 			<service name="ROM"    label="window_layout">     <parent label="window_layout"/>     </service>

repos/os/include/os/session_policy.h

@@ -15,6 +15,7 @@
 #define _INCLUDE__OS__SESSION_POLICY_H_
 
 #include <base/session_label.h>
+#include <base/log.h>
 #include <session/session.h>
 #include <util/arg_string.h>
 #include <util/xml_node.h>
@@ -199,6 +200,7 @@ class Genode::Session_policy : public Xml_node
 			try { return config.sub_node("default-policy"); }
 			catch (...) { }
 
+			warning("no policy defined for label '", label, "'");
 			throw No_policy_defined();
 		}