Parse /settle errors and only warn about them (#2004)

# Description
#1999 had the unfortunate
side effect that we spammed a few
[alerts](https://cowservices.slack.com/archives/C037PB929ME/p1697812387457109)
whenever the driver was not able to settle a solution.

# Changes
We now only `warn` whenever we can't parse the success response. We
didn't introduce, expect and parse any specific error format because
eventually drivers will be run externally and maybe not all of them run
a driver compliant with the error response spec all the time.
If we simply print the parsing error we'll get all the data we can get
out of the response anyway.

crates/autopilot/src/run_loop.rs

@@ -289,7 +289,7 @@ impl RunLoop {
                 Ok(()) => Metrics::settle_ok(driver),
                 Err(err) => {
                     Metrics::settle_err(driver, &err);
-                    tracing::error!(?err, driver = %driver.name, "settlement failed");
+                    tracing::warn!(?err, driver = %driver.name, "settlement failed");
                 }
             }
         }