Log when attempt to fetch quotes fails (#9723)

If the attempt to fetch quotes fails, an error message is now logged to
the console explaining why it failed. This makes it dramatically easier
to understand what's happening when quotes fail.

At some point later on we should also look specifically for errors we
expect to occur, and report to Sentry in unexpected cases. I've added a
TODO comment about this.

ui/app/ducks/swaps/swaps.js

@@ -402,8 +402,11 @@ export const fetchQuotesAndSetQuoteState = (history, inputValue, maxSlippage, me
     } catch (e) {
       // A newer swap request is running, so simply bail and let the newer request respond
       if (e.message === SWAPS_FETCH_ORDER_CONFLICT) {
+        log.debug(`Swap fetch order conflict detected; ignoring older request`)
         return
       }
+      // TODO: Check for any errors we should expect to occur in production, and report others to Sentry
+      log.error(`Error fetching quotes: `, e)
       dispatch(setSwapsErrorKey(ERROR_FETCHING_QUOTES))
     }