handle no-tty case in gum_func with our old/better error

[jhheider]

No description provided.

[what-the-diff]

• The function gum_func() was added.
• A new elif statement was added to the if-statement in line 214, which checks whether stdout is not a terminal (i.e., it's redirected). If so, then return 1 and exit the script with an error code of 1; otherwise continue executing as normal by returning 0 and exiting normally without any errors or warnings.