I would expect a break to be caught. Instead I get "The ebuild phase 'prepare' has exited unexpectedly. [..]" sys-apps/portage-2.1.9.49
Maybe we can use WIFSIGNALED to check if it exited due to a signal, and then output a better message, since it's obviously not "unexpected" in this case.
This is fixed in git: http://git.overlays.gentoo.org/gitweb/?p=proj/portage.git;a=commit;h=ab6d03f3813318f9ba49e75a52d42972e14cdcc7
This is fixed in 2.1.10.1 and 2.2.0_alpha39.