Add NoCreateError and fix a bug about issubclass

With nocreate=True, if page doesn't exist, API will return
missingtitle which is not recognized by PWB. This patch adds
NoCreateError to match with this missingtitle in this case.

This patch also fixes a bug when issubclass is used but the first
parameter is not a class, leading to an error
"TypeError: issubclass() arg 1 must be a class"

Change-Id: I11f80783c9febce51538e868a0b1edb42cce4ec3
Merge "Add NoCreateError and fix a bug about issubclass"