Return an error instead of asserting when exceeding
the number of available varying variables.

Change-Id: Ia4d76897b4b214a5b3bdd37cd420c0437faa216a
Reviewed-on: https://swiftshader-review.googlesource.com/1451Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>