flow + ipc[_*]: Style/readability: Use static_assert(false, "...") instead of `#error "..."`.

[ygoldfeld]

Style/readability: Use static_assert(false, "...") instead of #error "...".

Do this for Flow and Flow-IPC.

Recommend as much in doc-coding_style.cpp.

A few places already use it. It's just more convenient for multi-line error strings; plus it's vaguely more modern. The emitted errors are nicer.