WebResourceHelper: Validation should catch proposed deletion of WRs with dependencies

[dhedey]

Currently with the /removemissing command, there are no checks if the web resource has dependencies (eg it could be a form library or a web resource on a form), and instead the error is only exposed when the actions are being carried out.

This is quite a minor issue, as the deletion erroring at execution time is less of an issue than a create/update erroring, as the latter may result in only a subset of new code being released, causing functional issues.