Make exception about missing mediainfo more friendly

Before throwing, re-use the pageid of the file to avoid
exception saying "Entity '-1' ..."

Also don't show the KeyError traceback.

Change-Id: I1a130419477960856b6f976762a128d100eb43a6
---
M pywikibot/page/_wikibase.py
1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/pywikibot/page/_wikibase.py b/pywikibot/page/_wikibase.py
index 5141e00..7f098f8 100644
--- a/pywikibot/page/_wikibase.py
+++ b/pywikibot/page/_wikibase.py
@@ -433,8 +433,10 @@
                     data = self.file.latest_revision.slots['mediainfo']['*']
                 except NoPageError as exc:
                     raise NoWikibaseEntityError(self) from exc
-                except KeyError as exc:
-                    raise NoWikibaseEntityError(self) from exc
+                except KeyError:
+                    # reuse the reserved ID for better message
+                    self.id = 'M' + str(self.file.pageid)
+                    raise NoWikibaseEntityError(self) from None
 
                 self._content = jsonlib.loads(data)
                 self.id = self._content['id']

