linux/frida-helper-backend: handle process gone #357

Job	Run time
native (windows-x86_64, "windows-latest")	5m 11s
cross (windows-x86_64-mingw, --host=x86_64-w64-mingw32 --without-prebuilds=sdk:host, g++-mingw-w6...	3m 29s
manylinux (x86)	2m 41s
mobile (ios-arm64)	17s
cross (windows-x86-mingw, --host=i686-w64-mingw32 --without-prebuilds=sdk:host, g++-mingw-w64-i686)	2m 59s
manylinux (x86_64)	4m 11s
mobile (android-x86)	14s
cross (linux-mips, --host=mips-linux-gnu, g++-mips-linux-gnu)	2m 14s
native (windows-x86, "windows-latest")	5m 25s
manylinux (x86_64-musl)	2m 52s
mobile (android-x86_64)	14s
cross (linux-mipsel, --host=mipsel-linux-gnu, g++-mipsel-linux-gnu)	2m 5s
native (macos-x86_64, "macos-latest")	16s
manylinux (armhf)	2m 32s
mobile (android-arm)	14s
cross (linux-mips64, --host=mips64-linux-gnuabi64, g++-mips64-linux-gnuabi64)	3m 0s
native (macos-arm64, "macos-latest")	15s
manylinux (arm64)	3m 4s
mobile (android-arm64)	13s
cross (linux-mips64el, --host=mips64el-linux-gnuabi64, g++-mips64el-linux-gnuabi64)	2m 29s
native (linux-x86_64, "ubuntu-latest")	2m 36s
manylinux (arm64-musl)	3m 1s
native (linux-x86, "ubuntu-latest")	2m 26s
manylinux (mips)	2m 7s
qnx-armeabi	3m 6s
native (freebsd-arm64, ["self-hosted", "freebsd", "arm64"])	1s
manylinux (mipsel)	2m 10s
manylinux (mips64)	2m 45s
manylinux (mips64el)	2m 45s
	1h 4m 52s

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