Improve error message for exec mode

[TheElectronWill]

How to reproduce

Try to spawn and monitor a particular program my_app.py that exists in the current directory:

alumet-agent exec my_app.py

Error:

thread 'main' panicked at app-agent/src/main.rs:81:18:
error in child process: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Alumet is right, the correct command is:

alumet-agent exec ./my_app.py

but the error message should be improved. Also, could it detect such a case and suggest the right command?