file:// and ssh:// protocols do not allow for failed gets

A user on #ubuntu reported failure of debootstrap to fetch the Release file; hanging whilst trying to deal with an InRelease file that doesn't exist when using ssh:// protocol.

scp: /opt/repository/isi/archive/15.11.6/trusty/dists/trusty/InRelease: No such file or directory
mv: cannot stat '/tmp/tmp-cache/var/lib/apt/lists/partial/debootstrap.invalid_dists_trusty_InRelease': No such file or directory
sed: can't read /tmp/tmp-cache/var/lib/apt/lists/debootstrap.invalid_dists_trusty_InRelease: No such file or directory
scp: /opt/repository/isi/archive/15.11.6/trusty/dists/trusty/InRelease: No such file or directory
mv: cannot stat '/tmp/tmp-cache/var/lib/apt/lists/partial/debootstrap.invalid_dists_trusty_InRelease': No such file or directory
sed: can't read /tmp/tmp-cache/var/lib/apt/lists/debootstrap.invalid_dists_trusty_InRelease: No such file or directory
scp: /opt/repository/isi/archive/15.11.6/trusty/dists/trusty/InRelease: No such file or directory
mv: cannot stat '/tmp/tmp-cache/var/lib/apt/lists/partial/debootstrap.invalid_dists_trusty_InRelease': No such file or directory
sed: can't read /tmp/tmp-cache/var/lib/apt/lists/debootstrap.invalid_dists_trusty_InRelease: No such file or directory

This seems to be due to a problem in functions::just_get() for both ssh:// and file:// protocols where the 'get' is assumed to have succeeded and hard-codes a "return 0".

I think these should be returning the exit code of the scp and cp commands respectively.

Suggested patch attached

Affects 16.04 and 17.10. Apparently doesn't affect 14.04.