[IMPR] Add NoSiteLinkError for missing sitelinks

- Raise NoSiteLinkError instead of NoPageError when sitelink
is missing
- NoSiteLinkError derives from NoPageError for back-compat

Bug: T332341
Change-Id: Ied7796fdca1216064726d9049901ccb1f7b646cf
---
M pywikibot/exceptions.py
M pywikibot/page/_wikibase.py
2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/pywikibot/exceptions.py b/pywikibot/exceptions.py
index 36da84e..9cd518a 100644
--- a/pywikibot/exceptions.py
+++ b/pywikibot/exceptions.py
@@ -26,6 +26,7 @@
           |    +-- PageLoadRelatedError
           |    |    +-- InconsistentTitleError
           |    |    +-- InvalidPageError
+          |    |    +-- NoSiteLinkError
           |    +-- PageSaveRelatedError
           |    |    +-- EditConflictError
           |    |    |    +-- ArticleExistsConflictError
@@ -106,6 +107,7 @@
 
 PageLoadRelatedError: any exception which happens while loading a Page.
   - InconsistentTitleError: Page receives a title inconsistent with query
+  - NoSiteLinkError: ItemPage has no sitelink to given language
 
 PageSaveRelatedError: page exceptions within the save operation on a Page
 
@@ -396,6 +398,21 @@
         super().__init__(page)
 
 
+class NoSiteLinkError(PageLoadRelatedError, NoPageError):
+
+    """ItemPage has no sitelink to the given language."""
+
+    def __init__(self, page: 'pywikibot.page.ItemPage', lang: str) -> None:
+        """Initializer.
+
+        :param page: ItemPage that caused the exception
+        :param lang: language code of the queried sitelink
+
+        """
+        self.message = f'Item {{}} has no sitelink to language {lang!r}'
+        super().__init__(page)
+
+
 class SiteDefinitionError(Error):
 
     """Site does not exist."""
diff --git a/pywikibot/page/_wikibase.py b/pywikibot/page/_wikibase.py
index 3ce4424..d904039 100644
--- a/pywikibot/page/_wikibase.py
+++ b/pywikibot/page/_wikibase.py
@@ -31,6 +31,7 @@
     IsNotRedirectPageError,
     IsRedirectPageError,
     NoPageError,
+    NoSiteLinkError,
     NoWikibaseEntityError,
     WikiBaseError,
 )
@@ -1078,7 +1079,7 @@
         """
         Return the title for the specific site.
 
-        If the item doesn't have that language, raise NoPageError.
+        If the item doesn't have that language, raise NoSiteLinkError.
 
         :param site: Site to find the linked page of.
         :type site: pywikibot.Site or database name
@@ -1086,13 +1087,13 @@
         :param get_redirect: return the item content, do not follow the
                              redirect, do not raise an exception.
         :raise IsRedirectPageError: instance is a redirect page
-        :raise NoPageError: site is not in :attr:`sitelinks`
+        :raise NoSiteLinkError: site is not in :attr:`sitelinks`
         """
         if force or not hasattr(self, '_content'):
             self.get(force=force)
 
         if site not in self.sitelinks:
-            raise NoPageError(self)
+            raise NoSiteLinkError(self, site.lang)
 
         return self.sitelinks[site].canonical_title()
 

