Add an invalid notebook error (#1675)

* Add an invalid notebook error
Motivation:
  - As we are now service hundreds of thousands of notebooks a day, we are
    seeing a number of errors from Notebook rendering that really stem
    from notebooks not being properly formated. These show up in a number
    of errors such as the error handled in this pr, an assertion error,
    we have also seen key errors and others. This pr is a bit of a proposal
    to instead introduced a common exception that indicates the notebook
    is missing some required data that is needed to render itself.

* Update nbconvert/filters/markdown_mistune.py

Co-authored-by: Matthias Bussonnier <bussonniermatthias@gmail.com>

Co-authored-by: Matthias Bussonnier <bussonniermatthias@gmail.com>

nbconvert/filters/markdown_mistune.py

@@ -160,7 +160,10 @@ def image(self, src, title, text):
         attachment_prefix = 'attachment:'
         if src.startswith(attachment_prefix):
             name = src[len(attachment_prefix):]
-            assert name in attachments, "missing attachment: {}".format(name)
+
+            if not name in attachments:
+                raise InvalidNotebook("missing attachment: {}".format(name))
+
             attachment = attachments[name]
             # we choose vector over raster, and lossless over lossy
             preferred_mime_types = ['image/svg+xml', 'image/png', 'image/jpeg']

nbconvert/nbconvertapp.py

@@ -595,6 +595,9 @@ def initialize(self, argv=None):
     def default_export_format(self):
         return 'html'
 
+class InvalidNotebook(Exception):
+    pass
+
 #-----------------------------------------------------------------------------
 # Main entry point
 #-----------------------------------------------------------------------------