ENH: early stopping in replay scripts

[nbokulich]

related to #81

the replay scripts have comment lines and placeholders for unknown parameters and filepaths. This is great!

But this also creates a problem, namely that the scripts are then not executable "out of the box". This is fine, they should not be expected to be (nor is "touchless replay" necessarily even the goal here), but it could create issues if a user naively attempts to run a script that has not yet been adapted.

this could be partially fixed by including instructions for making the script executable (#81 )

but it would also be nice to insert an exit line or similar (with comments/instructions to remove after other edits are made), so that users do not attempt to run a broken provenance replay script.

[ChrisKeefe]

Great idea, @nbokulich! A couple minor clarifications:

• instructions for making scripts executable are already written to script headers. #81 targets end users who need to execute scripts that have already been prepared according to these instructions. I've clarified this at #81.
• this should probably raise (or echo) some kind of explanatory error message, and for bash scripts use a somewhat meaningful unique error code (i.e. more searchable than exit 1). There is limited standardization of error codes in bash, but I propose exit 66. 66 codes EX_NOINPUT in C's sysexits, generally indicating a missing or unreadable input file.