home-manager: exit with an error on build failure

Because 'set -e' has no effect on commands that run in an if condition,
the script was always exiting with no error when 'doBuild' failed.

As a bonus, $wrkdir is now always removed after building.

(cherry picked from commit a9d9fb5d75)

home-manager/home-manager

@@ -91,11 +91,13 @@ function doSwitch() {
     local wrkdir
     wrkdir="$(mktemp -d)"
 
-    if doBuild "$wrkdir/generation" ; then
-        "$wrkdir/generation/activate"
-    fi
+    local exitCode=0
+    doBuild "$wrkdir/generation" && "$wrkdir/generation/activate" || exitCode=1
 
+    # Because the previous command never fails, the script keeps running and
+    # $wrkdir is always removed.
     rm -r "$wrkdir"
+    return $exitCode
 }
 
 function doListGens() {