Deleting Datasette.app breaks the virtual environment, prevents startup

[simonw]

I used a temporary Datasette.app and then deleted it. In doing so it created a venv which then broke with this error:

datasette-app % /Users/simon/.datasette-app/venv/bin/pip\
  install \
  "datasette>=0.59a2" \
  "datasette-app-support>=0.7" \
  "datasette-vega>=0.6.2" \
  "datasette-cluster-map>=0.17.1" \
  "datasette-pretty-json>=0.2.1" \
  "datasette-edit-schema>=0.4" \
  "datasette-configure-fts>=1.1"
zsh: /Users/simon/.datasette-app/venv/bin/pip: bad interpreter: /Users/simon/.datasette-app/venv/bin/python3.9: no such file or directory

The app has no error handling around the pip install call so it just hung on the loading screen.

[simonw]

I need to catch this error and, if it occurs, delete and re-create the virtual environment.

(Another reason that I should persist details of the plugins the user had already installed so I can help them recover from this, refs #72)