Improve the FileManageActionsBean

Currently, when a file is uploaded, the FileManageActionsBean:

• calls createDocumentFromBlob() with the flag overwrite always true
• it doesn't always check the returned value (in the case of null returned there are cases in which should appear a NPE)
• it doesn't properly check all the cases of failures. For instance, if the overwrite wouldn't be required, the case of couldn't update the document because already exists falls in the situation "wrong_document_type"
  Moreover, the logic in default implementation (defaultCreate() in FileManagerService) is somehow wrong: there are only 2 branches
  if (overwrite && docModel != null) {
  }else{
  }
  so the case: "do not overwrite and the document existing" is falling in the second case, and a "new" document is created.

This fix could be dangerous, having impacts in other plugins, because until now they were never been called with flag overwrite on false.