add EtherscanError::PageNotFound variant

[mattsse]

As discovered here

foundry-rs/foundry#4302

if API is down Page Not Found can be returned.

TODO

• add PageNotFound variant here

  ethers-rs/ethers-etherscan/src/errors.rs

  Line 5 in e20bb71

  pub enum EtherscanError {

• add check here

  ethers-rs/ethers-etherscan/src/lib.rs

  Lines 210 to 215 in e20bb71

  	if is_blocked_by_cloudflare_response(res) {
  	EtherscanError::BlockedByCloudflare
  	} else if is_cloudflare_security_challenge(res) {
  	EtherscanError::CloudFlareSecurityChallenge
  	} else {
  	EtherscanError::Serde(err)