scaf should fall back to python3 if python command is not found

[clshaw01]

Currently, scaf errors out if python not found. If python is not found, try python3.

If neither is found, stop and give user a friendly error.

First check for python3, then python -- if neither, gracefully exit and ask user to install.

Example: python3 wasn't mapped to python, so it errored out:

/home/christine/.local/bin/scaf: line 129: python: command not found Waiting for authorization... /home/christine/.local/bin/scaf: line 125: python: command not found cat: code.txt: No such file or directory rm: cannot remove 'code.txt': No such file or directory Error: Access token was not retrieved.