JsonableError has two major benefits over json_error:
* It can be raised from anywhere in the codebase, rather than
being a return value, which is much more convenient for refactoring,
as one doesn't potentially need to change error handling style when
extracting a bit of view code to a function.
* It is guaranteed to contain the `code` property, which is helpful
for API consistency.
Various stragglers are not updated because JsonableError requires
subclassing in order to specify custom data or HTTP status codes.
Note that the documentation cannot fully use our macros, because
Uptime Robot requires an & of the end of the URL, because of how it
passes its payload.
Fixes#13854. Fixes#13939.
PIG208
Adam Birds