Error handling
Improve error handling in symbol_table.c
- Handle the errors at a central place (make sure to use same message-format as in parser)
- Add all errors (currently just the last one is printed)
- maybe "mcc_ast_symbol_table.c" should return table and errors
- correct filename in errormessage
- correct out for frpintf (currently "stderr" is used)
- use enum for different types of errormessages