Prohibit branch creation and basebackup at out of scope lsns

Out of scope LSNs include pre initdb LSNs, and LSNs prior to
latest_gc_cutoff.

To get there there was also two cleanups:
* Fix error handling in Execute message handler. This fixes behaviour
  when basebackup retured an error. Previously pageserver thread just
  died.
* Remove "ancestor" file which previously contained ancestor id and
  branch lsn. Currently the same data can be obtained from metadata file.
  And just the way we handled ancestor file in the code introduced the
  case when branching fails timeline directory is created but there is no data in it
  except ancestor file. And this confused gc because it scans
  directories. So it is better to just remove ancestor file and clean up
  this timeline directory creation so it happens after all validity
  checks have passed

zenith_utils/src/postgres_backend.rs

@@ -450,7 +450,21 @@ impl PostgresBackend {
             }
 
             FeMessage::Execute(_) => {
-                handler.process_query(self, unnamed_query_string.clone())?;
+                trace!("got execute {:?}", unnamed_query_string);
+                // xxx distinguish fatal and recoverable errors?
+                if let Err(e) = handler.process_query(self, unnamed_query_string.clone()) {
+                    let errmsg = format!("{}", e);
+
+                    warn!(
+                        "query handler for {:?} failed: {:#}",
+                        unnamed_query_string, e
+                    );
+                    self.write_message(&BeMessage::ErrorResponse(errmsg))?;
+                }
+                // NOTE there is no ReadyForQuery message. This handler is used
+                // for basebackup and it uses CopyOut which doesnt require
+                // ReadyForQuery message and backend just switches back to
+                // processing mode after sending CopyDone or ErrorResponse.
             }
 
             FeMessage::Sync => {