Windows: Check if the books' files are in use before deleting

Fixes #1240788 [Calibre refused to delete book](https://bugs.launchpad.net/calibre/+bug/1240788)

src/calibre/db/cache.py

@@ -1397,6 +1397,10 @@ class Cache(object):
             except:
                 path = None
             path_map[book_id] = path
+        if iswindows:
+            paths = (x.replace(os.sep, '/') for x in path_map.itervalues() if x)
+            self.backend.windows_check_if_files_in_use(paths)
+
         self.backend.remove_books(path_map, permanent=permanent)
         for field in self.fields.itervalues():
             try:

src/calibre/gui2/actions/delete.py

@@ -5,6 +5,7 @@ __license__   = 'GPL v3'
 __copyright__ = '2010, Kovid Goyal <kovid@kovidgoyal.net>'
 __docformat__ = 'restructuredtext en'
 
+import errno
 from functools import partial
 from collections import Counter
 
@@ -360,7 +361,17 @@ class DeleteAction(InterfaceAction):
             return
         next_id = view.next_id
         if len(to_delete_ids) < 5:
-            view.model().delete_books_by_id(to_delete_ids)
+            try:
+                view.model().delete_books_by_id(to_delete_ids)
+            except IOError as err:
+                if err.errno == errno.EACCES:
+                    import traceback
+                    fname = getattr(err, 'filename', 'file') or 'file'
+                    return error_dialog(self.gui, _('Permission denied'),
+                            _('Could not access %s. Is it being used by another'
+                            ' program? Click "Show details" for more information.')%fname, det_msg=traceback.format_exc(),
+                            show=True)
+
             self.library_ids_deleted2(to_delete_ids, next_id=next_id)
         else:
             self.__md = MultiDeleter(self.gui, to_delete_ids,