Prevent '500' error when using forbidden marker

If an existing image id was used as a marker, but
the user didn't have access to that image a '500'
error occurred. Return '400' instead.

Fixes bug 1178307.

Change-Id: Ib632297d944a19e18694fff154307a3bc6d5b625
Merge "Prevent '500' error when using forbidden marker"