Unreliable requests (synths and tokens) on "Convert" page

[drptbl]

Description:
There are two unreliable requests on "Convert" page:

"Convert" functionality doesn't work and "Select Token" modal is empty because of it.
Other than fixing this, part of this task should also be to improve error handling in such case on Kwenta to avoid such things (slippage -100%):

It wasn't working for more than 1-2 hours when I was testing it.

Reproduction steps:

1. Go to Kwenta "Convert" page (https://dev.kwenta.io/dashboard/convert)

Desktop (please complete the following information):

• OS: macOS Big Sur 11.2.2
• Browser: Chrome 89.0.4389.95

[evgenyboxer]

Hmm... @justinjmoses @clementbalestrat @andytcf any ideas why these requests sometimes fail?

[drptbl]

synths.snx.eth.link request is also used on synthetix website now; "Synths" page:
https://synthetix-website-git-feat-synths-page-synthetixio.vercel.app/synths
so if this url fails to respond, not only convert feature won't be working but also "Synths" page.

[jcmonte]

I think this was a one time event with ipfs/ens domains. I'm not able to replicate this, but now we use zapper token lists which aren't ipfs/ens so I think this issue can be closed

synths.snx.eth.link also has a fallback with synthetix-assets for this case

[jcmonte]

Also improved the -100% slippage handling in #347