Advanced debugging functionality on error #1

It could be nice to have an environment variable which could be set (e.g. UMBER_DEBUG) which provides an interactive console interface to the user on error or fatal error.

This interface could give context to the line which errored by reading the source file in question and providing pretty colourised output. It could also allow the user to print out running information about the program, such as the callstack, and perhaps even variable contents by integrating with LLDB/GDB (no clue how that would work of it's even possible at all).