It is dangerous to set this on a per-process basis. If we even
still want the ability to have observer errors not kill the
session (debatable) we should do so by having an option on the
session object that is set when the shiny server function runs.
Not only does this remove support for the option, but now we
will refuse to connect and will abort the session if the option
is set to a non-NULL value. This is to prevent apps from moving
forward with the assumption that their option will work, when
it really won't. (I doubt anyone is using it anyway, it's so
obscure...)
