Make sure important errors are always shown to the user

Summary:
This diff makes sure that important errors are always shown to the user, such as an xcode version mismatch.

Also made some small styling improvements

Reviewed By: jknoxville

Differential Revision: D19660337

fbshipit-source-id: 24dfb4d98bfdd1666164403f87b6a4e1dc915ddb

src/reducers/connections.tsx

@@ -43,6 +43,7 @@ export type FlipperError = {
   message: string;
   details?: string;
   error?: Error | string;
+  urgent?: boolean; // if true this error should always popup up
 };
 
 export type State = {