> What do you do about programming errors you cannot identify?

A programming language designer can only do so much for you. If you cannot identify errors in your programs on your own, consider switching to a different career.

> And what about programming errors that your process and your language causes?

Rigorous reasoning cannot possibly cause any problems. Rigorous reasoning helps identify and fix problems more often than not caused by prior non-rigorous reasoning.

> Programming language design is all about interactions with humans

Humans who need to reason rigorously in order to do things right, hence need to be pushed in this direction.