Refactor error and warning messages printing

– Harmonize location output for warnings so that it is the same as for
  errors (i.e. more explicit about what is the line number and the column number)
– Remove obsolete stuff for delayed error messages that should have been
  removed in db1f6c29.
– Factorize some printing logic with ParsingDriver::undeclared_model_variable_error().
– Simplify WarningConsolidation class