site.py: raise NoPage instead of Error

In site.movepage() raise NoPage instead of Error if no page exists.
This can be a problem for scripts that expect the proper exception.
E.g. movepages.py expects NoPage to be raised.

Added test in site_tests.py.
Also reordered imports in site_tests.py.

Change-Id: Ie89ec7dc8bd4ee99af4c8e51a779fedb52e098da
Merge "site.py: raise NoPage instead of Error"