Summary:
On Windows, the default action for things like division by zero and
segfaults is to pop up a Dr. Watson error reporting dialog if the exception
is unhandled by the user code.

This is a pain when we are SSHed into a Windows machine, or when we
want to debug a problem with gdb (gdb will get a first and second chance to
handle the exception, but if it doesn't the pop-up will show).

veh_excn provides two macros, `BEGIN_CATCH` and `END_CATCH`, which
will catch such exceptions in the entire process and die by
printing a message and calling `stg_exit(1)`.

Previously this code was handled using SEH (Structured Exception Handlers)
however each compiler and platform have different ways of dealing with SEH.

`MSVC` compilers have the keywords `__try`, `__catch` and `__except` to have the
compiler generate the appropriate SEH handler code for you.

`MinGW` compilers have no such keywords and require you to manually set t...