Extensions: Make it possible for permission parsing code to return a detailed error message.

extensions/common/permissions/api_permission.h

Index: extensions/common/permissions/api_permission.h
===================================================================
--- extensions/common/permissions/api_permission.h	(revision 247036)
+++ extensions/common/permissions/api_permission.h	(working copy)
@@ -204,8 +204,9 @@
   // Returns true if |rhs| is equal to this.
   virtual bool Equal(const APIPermission* rhs) const = 0;
 
-  // Parses the APIPermission from |value|. Returns false if error happens.
-  virtual bool FromValue(const base::Value* value) = 0;
+  // Parses the APIPermission from |value|. Returns false if an error happens
+  // and optionally set |error| if |error| is not NULL.
+  virtual bool FromValue(const base::Value* value, std::string* error) = 0;
 
   // Stores this into a new created |value|.
   virtual scoped_ptr<base::Value> ToValue() const = 0;