Add nice error messages

Currently when the markdown file is malformed, it just fails without giving any hint why. Adding high quality error messages would help here.

Start by locating the markdown parsing entry point in this static site generator and reproduce the failure with a malformed markdown file. Trace where parsing errors are discarded or surfaced, then define what information a useful error should provide. Done means malformed input reports a clear explanation instead of failing without a hint.

Written by the indexing model from the issue text.