Issue 1282 - Show error message when user tries to block whitelisted elements

_locales/en_US/messages.json

 {
   "add": {
     "description": "This is the label for the 'Add' buttons.",
     "message": "Add"
   },
   "add_domain_button": {
     "message": "Add domain"
   },
   "add_filter_button": {
     "message": "Add filter"
@@ skipping 188 matching lines @@
     }
   },
   "line": {
     "message": "Line $number$",
     "description": "Prepended to error messages to indicate the line number (e.g. \"Line 42: Some error on this line\")",
     "placeholders": {
       "number": {
         "content": "$1"
       }
     }
+  },
+  "cannot_block_because_exception_rule": {
+        "description": "Error message shown by the \"Block element\" dialog, when the selected element is whitelisted by a user-defined exception rule",
+    "message": "This element cannot be blocked because the exception rule $rule$",
+    "placeholders": {
+      "rule": {
+        "content": "$1"
+      }
+    }
+  },
+  "cannot_block_because_exception_rule_with_origin": {
+        "description": "Error message shown by the \"Block element\" dialog, when the selected element is whitelisted by an exception rule in a subscribed filter list",
+    "message": "This element cannot be blocked because the exception rule $rule$ in the $origin$ filter list",
+    "placeholders": {
+      "rule": {
+        "content": "$1"
+      },
+      "origin": {
+        "content": "$2"
+      }
+    }
+  },
+  "cannot_block_because_acceptable_ads": {
+        "description": "Error message shown by the \"Block element\" dialog, when the selected element is an Acceptable Ad™, and therfore can't be blocked while the \"Allow some non-intrusive advertising\" option is enabled",
+    "message": "This element isn't blocked because the option to allow some non-instrusive ads is currently enabled"
+  },
+  "cannot_block_because_domain_whitelisted": {
+        "description": "Error message shown by the \"Block element\" dialog, when selecting an element in a frame showing a website, the user has disabled Adblock Plus for",
+    "message": "Elements in this frame cannot be blocked while you have $domain$ whitelisted",
+    "placeholders": {
+      "domain": {
+        "content": "$1"
+      }
+    }
   }
 }