Fix handling apt install errors in install script

Logging the error code reset it (as echo executed successfully), so save
it first.

Also check for the failure of the second attempt to run apt too.

build/tools/before_install.sh

@@ -22,9 +22,10 @@ case $(uname -s) in
                 # Disable some (but not all) output.
                 $SUDO apt-get -q -o=Dpkg::Use-Pty=0 "$@"
 
-                echo "-> Done with $?"
+                rc=$?
+                echo "-> Done with $rc"
 
-                return $?
+                return $rc
             }
 
             if [ "$wxUSE_ASAN" = 1 ]; then
@@ -81,13 +82,15 @@ case $(uname -s) in
 
             if ! run_apt install -y $pkg_install $dbgsym_pkgs; then
                 if [ -z "$dbgsym_pkgs" ]; then
-                    exit $?
+                    exit 1
                 fi
 
                 # Retry without dbgsym packages that currently fail to install
                 # under Ubuntu Focal (20.04).
                 echo 'Installing with dbgsym packages failed, retrying without...'
-                run_apt install -y $pkg_install
+                if ! run_apt install -y $pkg_install; then
+                    exit 1
+                fi
             else
                 touch wx_dbgsym_available
             fi