This request header contains the context of the layer
targeted for a failed fragment update, serialized as JSON.
A fragment update is considered failed if the server responds with a
status code other than 2xx, but still renders HTML.
Server-side code is free to render different HTML for different contexts.
For example, you might prefer to not render a site navigation for overlays.
The user may choose to not send this header by configuring
up.network.config.requestMetaKeys.
Example
X-Up-Fail-Context: { "context": "Choose a company contact" }
