Handle out of memory on serialization better

[othercorey]

Whenever there is a type that can't be serialized, ToolbarService throws a confusing out of memory fatal error.

Most users don't know what ToolbarService is trying to do which makes it hard to debug.

[tbelknapsgs]

It is extremely confusing! In fact, the DebugKit itself needs some debugging work: any time it gets data it doesn't understand it seems to dump with an error. For example: I incorrectly formatted my Entity class's $_virtual array (I thought it was supposed to look like the $_accessible array) and I got error messages from DebugKit about an Entity formatting issue but it took forever to figure out that it was DebugKit, not CakePHP proper, choking on my error.