Improve error messages; Add `--py2hwsw_docs` argument; Initial py2 user guide; Fix linter;

[arturum1]

• Improve py2 error messages; Add more checks for correct user input;
• Add --py2hwsw_docs argument to generate a py2 specific document directory.
• Initial implementation of py2 user guide document. Use following commands to build it: py2hwsw --py2hwsw_docs; make -C py2hwsw_docs/document/ build
• Add support for common_src directory in py2 verilog linter.