preprocess.py: preprocessor functions now yield source file name and error location instead of just error location

After the parsing stage, line and column of error messages are now being determined when adding an error to the root-node, already. No need to call Error.adjust_error_locations() on later tree processing stages

compiler.py - fallback_compiler is now more tolerant w.r.t. compilation methods that manipulate the parent tree; this means less need for finalizers

error.py: Refactoring: Error-constants are now defined on the top-level of error.py, rather than as class Attributes of class Error

compile.py: better error message when fallback_compiler()-method fails; ebnf.py: added on_ZOMBIE__()-method to better handle faulty syntax-trees.