feat: display last error when definition/config load fails

Contains a breaking internal API change to `errActions.newThrownErr`, which will require a MINOR version bump. Swagger-Editor does not depend on the old usage style.

src/core/plugins/download-url.js

@@ -26,7 +26,7 @@ export default function downloadUrlPlugin (toolbox) {
       function next(res) {
         if(res instanceof Error || res.status >= 400) {
           specActions.updateLoadingStatus("failed")
-          return errActions.newThrownErr( new Error(res.statusText + " " + url) )
+          return errActions.newThrownErr( new Error((res.message || res.statusText) + " " + url) )
         }
         specActions.updateLoadingStatus("success")
         specActions.updateSpec(res.text)

src/core/plugins/err/actions.js

@@ -7,10 +7,10 @@ export const NEW_SPEC_ERR_BATCH = "err_new_spec_err_batch"
 export const NEW_AUTH_ERR = "err_new_auth_err"
 export const CLEAR = "err_clear"
 
-export function newThrownErr(err, action) {
+export function newThrownErr(err) {
   return {
       type: NEW_THROWN_ERR,
-      payload: { action, error: serializeError(err) }
+      payload: serializeError(err)
   }
 }