Feature: fire a per-script callback if a "test" fails

[getify]

Again, very tricky to do cleanly, but let's investigate if you could specify a "test" to be performed after a script finished executing, and if the test fails, then fire an error callback.

Not sure if it's possible to "abort" the chain at this point (which is what would be expected, likely), but at least you can know it happened.

Also, a variation of this might be to provide a timeout "test" where if the script takes too long, notify the callback of this potential "error".

[getify]

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