Return Promise from `editRpc` thunk

`editRpc` 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

@@ -1461,24 +1461,29 @@ export function updateAndSetCustomRpc (newRpc, chainId, ticker = 'ETH', nickname
 }
 
 export function editRpc (oldRpc, newRpc, chainId, ticker = 'ETH', nickname, rpcPrefs) {
-  return (dispatch) => {
+  return async (dispatch) => {
     log.debug(`background.delRpcTarget: ${oldRpc}`)
-    background.delCustomRpc(oldRpc, (err) => {
-      if (err) {
-        log.error(err)
-        return dispatch(displayWarning('Had a problem removing network!'))
-      }
-      dispatch(setSelectedToken())
-      background.updateAndSetCustomRpc(newRpc, chainId, ticker, nickname || newRpc, rpcPrefs, (err) => {
-        if (err) {
-          log.error(err)
-          return dispatch(displayWarning('Had a problem changing networks!'))
-        }
-        dispatch({
-          type: actionConstants.SET_RPC_TARGET,
-          value: newRpc,
-        })
-      })
+    try {
+      promisifiedBackground.delCustomRpc(oldRpc)
+    } catch (error) {
+      log.error(error)
+      dispatch(displayWarning('Had a problem removing network!'))
+      return
+    }
+
+    dispatch(setSelectedToken())
+
+    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,
     })
   }
 }