Improve `libtock_runtime`'s error handling if it can't find an assembly toolchain.
Improve `libtock_runtime`'s error handling if it can't find an assembly toolchain.

Previously, if `libtock_runtime`'s build script was unable to find a toolchain to build its external assembly with, its build would succeed then the process binary would fail to link. This change makes `libtock_runtime`'s build script panic with a useful error message instead.

tock/libtock-rs