WebStyle: migrate to exception logging

• Migrates error logging to new exception-based technique. (addresses #580)
• Deletes 'errors' and 'warnings' variables in function 'page()', and delete code that uses these variables.
• Updates comments on function 'page()'
• Includes parameters 'errors' and 'warnings' in the function 'page()' header inicialized to 'None' instead of '[]'.