Check goo script arguments to prevent data loss.

[tkphd]

Often, the Instructor and Learners will run goostats.sh without arguments. The resulting usage message unhelpfully requests two files -- without warning that the second will be written into!

This PR updates both "goo" scripts to check file arguments for existence before continuing.

goostats.sh: If $1 is absent, or $2 is present, exit 2 (invalid arguments).

goodiff.sh: If $1 or $2 is absent, exit 2 (invalid arguments).

"Usage" also updated to use $0 rather than hard-coding the script names.

[github-actions]

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⏱️ Updated at 2025-08-18 11:46:51 +0000

[bkmgit]

Thanks for this. Any reason for changing the return code to 0 from 1 when the incorrect number of arguments are given?