I usually move this up in an ETL process into a function I usually call "Pre_Flight_Checklist." Over the years this has gotten its own helper functions. For example, if I am going to use a file, I have function that not only checks for the existence of the file, but that it is a file. This function can also be fed an expected size range and date if "freshness" is expected. If something is out of whack, an error message will mention that a file was expected at /Adirectory/Bdirectory/Cdirectory/filename.ext, but only /Adirectory/Bdirectory was found, nothing under it. I believe in overly-explanatory error messages.

Now, I do run the risk of having things change out from under me, but putting all of the defensiveness up front allows the happy path and some alternatives to be more succinct in the code.

I keep finding new things to be paranoid about, though!