While this is classed as a 'string' in the spec,
the provided value must be an integer in order
for pagination to work. Thus any non-integer value
must be rejected.

Adds a test for pagination for /notifications. We also add a regression
test for the 'from' parameter not being an integer.

I wanted to break this up into two commits, but PyCharm made it hard.

I'm not sure why it was there.

Instead of telling the client that they should have used an integer -
which could be confusing as the spec says that the query parameter must
be a string - we simply tell the client that their "from" token is
unknown to us.

Co-authored-by: Erik Johnston <erikj@element.io>