client: Return url.Error from HTTPRemoteStore.get
client: Return url.Error from HTTPRemoteStore.get

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

diogomonica/gotuf,cyli/gotuf,dmcgowan/gotuf,runcom/gotuf,endophage/gotuf,theupdateframework/go-tuf