Force text error template when exceptions occur

Rails by default shows nice error pages, with a lot of markup, css and
javascript. This is cool when you visit the page in the browser, but
not helpful when you consume the app as an API. Rails' HTML response
then makes it really hard find the error when it's displayed as part of
the exception message.

But Rails will render a text-based exception when doing AJAX requests.
By setting the `X-Requested-With` header we can make Rails show the
text template for all requests.

Note that there's still some markup in the error. This is a Rails bug,
addressed in rails/rails#22172.

I feel this is not a great solution. I'm planning on making a PR on
Rails to show the text-template for all non-HTML requests.

app/controllers/application_controller.rb

@@ -6,6 +6,11 @@ class BadRequest < StandardError; end
     head :bad_request
   end
 
+  before_action do
+    # Force Rails to show text-error pages
+    request.env['HTTP_X_REQUESTED_WITH'] = 'XMLHttpRequest'
+  end
+
 private
   def respond_with_command_error(error)
     render status: error.code, json: error