Do not error out when removing old cover if there are images with invalid URLs in the HTML

src/calibre/ebooks/oeb/transforms/metadata.py

@@ -171,11 +171,14 @@ class MergeMetadata(object):
         for item in self.oeb.spine:
             try:
                 images = XPath('//h:img[@src]')(item.data)
-            except:
-                images = []
+            except Exception:
+                images = ()
             removed = False
             for img in images:
+                try:
                     href = item.abshref(img.get('src'))
+                except Exception:
+                    continue  # Invalid URL, ignore
                 if href == cover_item.href:
                     img.getparent().remove(img)
                     removed = True
@@ -195,6 +198,3 @@ class MergeMetadata(object):
                         ' the cover image'%item.href)
                 self.oeb.spine.remove(item)
                 self.oeb.manifest.remove(item)
-
-
-