Fixes #939.

Since smash.load uses a separate context, any exceptions it throws use a
different Error class, and thus the assert.throws fails. So, instead, just allow
any exception type to be thrown.

This corrects the handling of lines that are long enough to have two
visible or invisible endpoints, but still cross the small circle and
thus have an invisible or visible intermediate segment.

Fixes #1127.

This allows all listeners with a particular name to be removed.

Fixes #880.