This includes explanation and context for non-obvious cases and
shortens debug cycles when just-in-time debugging is not available
(like on Linux) by immediately printing description of the assert
as well as symbolized backtrace.
Without resources, it makes no sense anymore to keep these in
separate subdirectories: unixutil* is shared between Cocoa
and GTK ports, and gloffscreen* should be shared between all ports
(but it's still platform-specific).
