Ex handlers: filter out invalid ex-data

[andrewberls]

Overseer uses user-supplied ex-data as additional metadata when sending errors to Sentry. However, values are serialized as JSON when sending over the wire, so we need to ensure we only attempt to send serializable values in the first place. We've seen bugs where, e.g. prismatic/schema fails validation on a datatype and includes the value in the ex-data, causing serialization errors.

Before, something like

(throw (ex-info "" {:foo 1 :bar (Object.)}))

would cause the failure. Now, invalid keys/values are simply stripped out of the data map.

[elliot42]

:+1: pending the "use nil to represent and check 'nothing was here' instead of empty map"

[elliot42]

:+1: