Clarify the "Non accessible element" error

This error happens, among others, when trying to set an element as if it was a
parameter. The error message was not very helpful.

Previous error message:
        Non accessible element

New error message:
       Can't set /A/b/c because it is not a parameter

Signed-off-by: David Wagner <david.wagner@intel.com>
diff --git a/parameter/ConfigurableElement.cpp b/parameter/ConfigurableElement.cpp
index c23c7e9..eb66dd8 100644
--- a/parameter/ConfigurableElement.cpp
+++ b/parameter/ConfigurableElement.cpp
@@ -139,7 +139,7 @@
 
     if (!pStrChildName) {
 
-        parameterAccessContext.setError("Non accessible element");
+        parameterAccessContext.setError((bSet ? "Can't set " : "Can't get ") + pathNavigator.getCurrentPath() + " because it is not a parameter");
 
         return false;
     }