gnosis / dex-services / 4726

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travis-pro

Single error type (#1299)

This PR introduces a common `RejectionReason` type that implements `Reject` and `Into<Rejection>` so that it can be used with the usual Rust `?` sugar, allowing for differentiation between rejection reasons without having to create a new type for each one.

### Test Plan

Run the price estimator and check that the rejections still work:
```
$ cargo run -p price-estimator
...
$ curl -s 'http://localhost:8080/api/v1/markets/9-7/estimated-buy-amount/1?atoms=false' | jq
{
  "message": "requested base units for token with missing ERC20 info"
}
$ curl -s 'http://localhost:8080/api/v1/markets/9-DAI/estimated-buy-amount/1?atoms=false' | jq
{
  "message": "token symbol or address not found"
}
```

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