Part of composer table edit work (bug 20973): Return NS_TABLELAYOUT_CELL_NOT_FOUND instead of error when a cell isn't found at a cellmap location. r=mjudge.

layout/html/table/src/nsTableFrame.cpp

@@ -4496,7 +4496,7 @@ CELL_FOUND:
           col = 0;
         }
       }
-      return NS_ERROR_FAILURE; // Some other error 
+      return NS_TABLELAYOUT_CELL_NOT_FOUND; // We didn't find a cell
     }
   }
 

layout/tables/nsTableFrame.cpp

@@ -4496,7 +4496,7 @@ CELL_FOUND:
           col = 0;
         }
       }
-      return NS_ERROR_FAILURE; // Some other error 
+      return NS_TABLELAYOUT_CELL_NOT_FOUND; // We didn't find a cell
     }
   }