(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
`snprintf` returns the number of characters that would have been written
had the buffer been large enough.
It's a common trick to call `snprintf(nullptr, 0, ...)` to measure how
large a buffer has to be.
Thus the return value is not zero but fourteen.
