[libc++abi] Reword uncaught exception termination message

When we terminate due to an exception being uncaught, libc++abi prints
a message saying "terminating with uncaught exception [...]". This patch
changes that to say "terminating due to uncaught exception [...]" instead,
which is a bit clearer. Indeed, I've seen some people being confused and
thinking that libc++abi was the component throwing the exception.

Differential Revision: https://reviews.llvm.org/D125245