Add error at startup when requiring an unknown bundle in deployment fragment

Steps to reproduce:

• in the deployment-fragment file of webapp-base, for instance, add the following:

<require>foo</require>

• deploy the jar on a tomcat server and start

=> the deployment fragment instructions are not taken into account for this jar (producing a lot of visible error on the default page)

=> there should be a very visible error log at startup about this missing requirement (as it's done for missing requirements on runtime extention points)