[WIP] assert context offset exception is Throwable

[sasezaki]

At #15, introduced array shape context: array{exception?: \Throwable}.
But I missed asserting for this.

Regarding #17, I'm wondering if I should change it to context: array<array-key, mixed>

[derrabus]

The assertion you're proposing would be off-spec, as the specification does not forbid storing non-throwables under the exception key. Your application could chose to convert an exception into an array before logging it or even store something completely different there.