Please bear in mind that all what I am about to suggest is from the top of my mind, therefore not tested, and may be wrong, but hey it doesn’t hurt for you to give it a try 
def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
send_resp(conn, conn.status, "Something went wrong")
end
If I am not mistake the above will override the call to the Error view at lib/yourapp_web/views/error_view.ex.
I think that this will return it in json:
# You may need to use the Jason library if returning only the map doesn't work.
# You also want to put the `application/json` header in the `conn`
send_resp(conn, conn.status,%{error: conn.status, message: "Something went wrong"})
Maybe you can simplify the above to use in your router:
# file: lib/yourapp_web/router.ex
pipeline :api do
plug :accepts, ["json"],
plug :handle_errors
end
# you may need to pattern match in `conn.status` to make this work
def handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
# You may need to use the Jason library if returning only the map doesn't work.
# You also want to put the `application/json` header in the `conn`
send_resp(conn, conn.status,%{error: conn.status, message: "Something went wrong"})
end
EDIT: On a second though I think this isn’t reflecting the intended use from the Plug.ErrorHandler
An Alternative that you can try is to change the Error view to always return a JSON object:
defmodule YourAppWeb.ErrorView do
use YourAppWeb, :view
require Logger
# If you want to customize a particular status code
# for a certain format, you may uncomment below.
def render(template, assigns) do
_json_error(template, assigns)
end
# By default, Phoenix returns the status message from
# the template name. For example, "404.html" becomes
# "Not Found".
def template_not_found(template, assigns) do
_json_error(template, assigns)
end
defp _json_error(template, assigns) do
Logger.info("Error template: #{template}")
# render("500.html", assigns)
# I am not sure, but I think you can just return a Map, otherwise just try to use the Jason library.
%{
error: 400, # You need to extract it from assigns or template name
maessage: "Whoops!"
}
end
end
