Set title when showing PDF.js error page

Fixes #3894

doc/changelog.asciidoc

@@ -116,6 +116,8 @@ Fixed
 - The security fix in v1.3.3 caused URLs with ampersands
   (`www.example.com?one=1&two=2`) to send the wrong arguments when clicked on
   the `qute://history` page.
+- Crash when opening a PDF page with PDF.js enabled (on QtWebKit), but no
+  PDF.js installed.
 
 Removed
 ~~~~~~~

qutebrowser/browser/webkit/webpage.py

@@ -212,7 +212,8 @@ class BrowserPage(QWebPage):
             page = pdfjs.generate_pdfjs_page(reply.url())
         except pdfjs.PDFJSNotFound:
             page = jinja.render('no_pdfjs.html',
-                                url=reply.url().toDisplayString())
+                                url=reply.url().toDisplayString(),
+                                title="PDF.js not found")
         self.mainFrame().setContent(page.encode('utf-8'), 'text/html',
                                     reply.url())
         reply.deleteLater()