pdfjs: throw PDFJSNotFound from None

Otherwise the stacktrace might be confusing since it will show the
FileNotFoundException as the causing error, which is not true (it just
happens to be the last checked place).

The .path attribute was added so that we still have the requested path
in the error log.

See #1280.

qutebrowser/browser/pdfjs.py

@@ -29,9 +29,16 @@ from qutebrowser.utils import utils
 
 class PDFJSNotFound(Exception):
 
-    """Raised when no pdf.js installation is found."""
+    """Raised when no pdf.js installation is found.
 
-    pass
+    Attributes:
+        path: path of the file that was requested but not found.
+    """
+
+    def __init__(self, path):
+        self.path = path
+        message = "Path '{}' not found".format(path)
+        super().__init__(message)
 
 
 def generate_pdfjs_page(url):
@@ -129,7 +136,7 @@ def get_pdfjs_res_and_path(path):
         try:
             content = utils.read_file(res_path, binary=True)
         except FileNotFoundError:
-            raise PDFJSNotFound
+            raise PDFJSNotFound(path) from None
 
     try:
         # Might be script/html or might be binary