Fix rate limit error code in fetch.ts

[SebastienGllmt]

According to the HTML list of error codes, 429 is what is returned on rate limiting errors (empirically, Arbitrum's RPC node also uses 429)

I believe the current value is a mistake and was meant as a placeholder, given 418 is the joke error code 418 I'm a teapot

I could be wrong and maybe some service returns this value. Strangely, this 418 value persists all the way back since 2017 when the file was originally created, so I don't know the context for this this was used