missing package breaks [package forget]

% package ifneeded foo 1.0 grok
% package forget notthere foo
% package ifneeded foo 1.0
grok

The docs for [package forget] say nothing about a
missing package silently terminating the "forgetting"
operation, but that's what happens.

Sigh. I still think the better way to handle this
was to force [package forget] to accept exactly one
package name as an argument (Bug 218410).