launch_functional_tests: Fail when tested-browsers don't exist and is required

Otherwise the script fails silently (because the sed command on the line below
fails but as it is launched in an external shell this doesn't impact the current
one.

browser/test/launch_functional_tests

@@ -7,6 +7,9 @@ fi
 if  [ "$1" = "--browsers-from-file" ]
 then
     shift
+    if [ ! -f ./tested-browsers ]; then
+        exit 1
+    fi
     BROWSER_LIST=`sed "s#^#$FUNCTIONAL_CLIENT_BROWSER_DIR#" ./tested-browsers`
 else
     BROWSER_LIST=$FUNCTIONAL_CLIENT_BROWSER_PATH