`source uv_env_setup.sh FAIL` closes the user's terminal

[zaz]

Describe the bug
Following the setup instructions in the Readme but specifying an incorrect platform closes the user's terminal.

To Reproduce

source uv_env_setup.sh FAIL

Expected behavior

❌ Error: Invalid platform 'FAIL'. Use: cpu, cu118, or cu121.

Actual behavior

Closes the user's terminal.
That's a bit aggressive.

Technical details
This is because we exit instead of returning. Tested on Linux.

PR to follow.