evolve: handle invalid obsmarkers in the `debugobsconvert`
User can now recover from such situation, The error message is also changed to
point the `debugobsconvert` commands.
all: tutorials/tutorial.rst static/logo-evolve.ico
sphinx-build . ../html/
tutorials/tutorial.rst: tutorials/tutorial.t
python test2rst.py tutorials/
static/logo-evolve.ico: static/logo-evolve.svg
convert -resize 36x36 static/logo-evolve.svg static/logo-evolve.ico