[coverage]Quote user value in error msgs

Some pfw bug induce empty criterion value,
when log it is dificult to see the the empty
value.

Add quotes around values.

before: Error in criterion
After : Error in criterion ""

Signed-off-by: Kevin Rocard <kevin.rocard@intel.com>
diff --git a/tools/coverage/coverage.py b/tools/coverage/coverage.py
index 1df02ae..f6f7789 100755
--- a/tools/coverage/coverage.py
+++ b/tools/coverage/coverage.py
@@ -59,11 +59,11 @@
 
 class ChildNotFoundError(ChildError):
     def __str__(self):
-        return "Unable to find the child %s in %s" % (self.child, self.parent)
+        return 'Unable to find the child "%s" in "%s"' % (self.child, self.parent)
 
 class DuplicatedChildError(ChildError):
     def __str__(self):
-        return "Add existing child %s in %s." % (self.child, self.parent)
+        return 'Add existing child "%s" in "%s".' % (self.child, self.parent)
 
 class Element():
     """Root class for all coverage elements"""
@@ -109,7 +109,7 @@
             if child.getName() == childName :
                 return child
 
-        self.debug("Child %s not found" % childName, logging.ERROR)
+        self.debug('Child "%s" not found' % childName, logging.ERROR)
 
         self.debug("Child list :")