Return Promise from `updateAndSetCustomRpc` thunk (#8440)

`updateAndSetCustomRpc` returned a thunk that didn't return a Promise,
despite doing async work. It now returns a Promise.

In the one place where this is used, it didn't seem important to update
the callsite to block on this finishing. Only one call followed it in
the event handler, and it didn't seem to depend on this.

ui/app/store/actions.js

@@ -1445,18 +1445,21 @@ export function setPreviousProvider (type) {
 }
 
 export function updateAndSetCustomRpc (newRpc, chainId, ticker = 'ETH', nickname, rpcPrefs) {
-  return (dispatch) => {
+  return async (dispatch) => {
     log.debug(`background.updateAndSetCustomRpc: ${newRpc} ${chainId} ${ticker} ${nickname}`)
-    background.updateAndSetCustomRpc(newRpc, chainId, ticker, nickname || newRpc, rpcPrefs, (err) => {
-      if (err) {
-        log.error(err)
-        return dispatch(displayWarning('Had a problem changing networks!'))
+
+    try {
+      await promisifiedBackground.updateAndSetCustomRpc(newRpc, chainId, ticker, nickname || newRpc, rpcPrefs)
+    } catch (error) {
+      log.error(error)
+      dispatch(displayWarning('Had a problem changing networks!'))
+      return
     }
+
     dispatch({
       type: actionConstants.SET_RPC_TARGET,
       value: newRpc,
     })
-    })
   }
 }