[Flang] Ported test_errors.sh to Python

To enable Flang testing on Windows, shell scripts have to be ported to Python. In this patch the "test_errors.sh" script is ported to python ("test_errors.py"). The RUN line of existing tests was changed to make use of the python script.

Used python regex in place of awk/sed.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D107575