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/standalone/layout.jsx

@@ -16,7 +16,7 @@ export default class StandaloneLayout extends React.Component {
   }
 
   render() {
-    let { getComponent, specSelectors } = this.props
+    let { getComponent, specSelectors, errSelectors } = this.props
 
     let Container = getComponent("Container")
     let Row = getComponent("Row")
@@ -27,6 +27,8 @@ export default class StandaloneLayout extends React.Component {
     const OnlineValidatorBadge = getComponent("onlineValidatorBadge", true)
 
     const loadingStatus = specSelectors.loadingStatus()
+    const lastErr = errSelectors.lastError()
+    const lastErrMsg = lastErr ? lastErr.get("message") : ""
 
     return (
 
@@ -43,12 +45,16 @@ export default class StandaloneLayout extends React.Component {
           <div className="info">
             <div className="loading-container">
               <h4 className="title">Failed to load API definition.</h4>
-            </div>
+              <p>{lastErrMsg}</p>
+            </div>            
           </div>
         }
         { loadingStatus === "failedConfig" &&
           <div className="info" style={{ maxWidth: "880px", marginLeft: "auto", marginRight: "auto", textAlign: "center" }}>
-            <h4 className="title">Failed to load remote configuration.</h4>
+            <div className="loading-container">
+              <h4 className="title">Failed to load remote configuration.</h4>
+              <p>{lastErrMsg}</p>
+            </div>
           </div>
         }
         { !loadingStatus || loadingStatus === "success" && <BaseLayout /> }