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

test_runner/batch_others/test_branch_behind.py

@@ -100,23 +100,18 @@ def test_branch_behind(zenith_simple_env: ZenithEnv):
     assert cur.fetchone() == (1, )
 
     # branch at pre-initdb lsn
-    #
-    # FIXME: This works currently, but probably shouldn't be allowed
-    try:
+    with pytest.raises(Exception, match="invalid branch start lsn"):
         env.zenith_cli(["branch", "test_branch_preinitdb", "test_branch_behind@0/42"])
-        # FIXME: assert false, "branch with invalid LSN should have failed"
-    except subprocess.CalledProcessError:
-        log.info("Branch creation with pre-initdb LSN failed (as expected)")
 
     # check that we cannot create branch based on garbage collected data
     with closing(env.pageserver.connect()) as psconn:
         with psconn.cursor(cursor_factory=psycopg2.extras.DictCursor) as pscur:
-            # call gc to advace latest_gc_cutoff
+            # call gc to advace latest_gc_cutoff_lsn
             pscur.execute(f"do_gc {env.initial_tenant} {timeline} 0")
             row = pscur.fetchone()
             print_gc_result(row)
 
-    with pytest.raises(Exception, match="(we might've already garbage collected needed data)"):
+    with pytest.raises(Exception, match="invalid branch start lsn"):
         # this gced_lsn is pretty random, so if gc is disabled this woudln't fail
         env.zenith_cli(["branch", "test_branch_create_fail", f"test_branch_behind@{gced_lsn}"])
 

test_runner/batch_others/test_readonly_node.py

@@ -86,6 +86,6 @@ def test_readonly_node(zenith_simple_env: ZenithEnv):
     assert cur.fetchone() == (1, )
 
     # Create node at pre-initdb lsn
-    with pytest.raises(Exception, match='extracting base backup failed'):
+    with pytest.raises(Exception, match="invalid basebackup lsn"):
         # compute node startup with invalid LSN should fail
-        env.zenith_cli(["pg", "start", "test_branch_preinitdb", "test_readonly_node@0/42"])
+        env.zenith_cli(["pg", "start", "test_readonly_node_preinitdb", "test_readonly_node@0/42"])