Basic error/exception object.

error.hpp

+namespace Sass {
+  
+  struct Error {
+    enum Type { read, write, syntax, evaluation };
+    
+    size_t line_number;
+    string file_name;
+    string message;
+    
+    Error(size_t line_number, string file_name, string message)
+    : line_number(line_number), file_name(file_name), message(message)
+    { }
+
+  };
+  
+}
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