Robustness while reading py modules from beacon

Closes #4120 (closed)

When corrupted python files was part of the beacon, the import was failing.

One of the big problem was that the full tree was read. So any corrupted files was able to break every sessions.

This PR

• Skip the wrong files (+warning in Beacon side)
  • Previously the server was failing, and then the client was timouting
• Display a more accurate exception for load_script
  • The error message was a python path while the user was requesting a file name