Bug 23888: (follow-up) Wrap English string in translation function

Bug 23888: (follow-up) Wrap English string in translation function

This patch makes alert added in subscription-add.js translatable using
the new __() function.

To test, apply the patch and confirm that the alert still works
correctly when submitting the subscription edit form with an invalid
vendor id.

Test that the string can be translated (using fr-Fr for example):

1. cd misc/translator && ./translate update fr-FR
2. Translate strings in misc/tranlator/po/fr-FR-messages-js.po
3. cd misc/translator && ./translate install fr-FR

Switch to the fr-FR translation and confirm the the alert shows your
translated string.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>