qa: Include ignored errors in RPC connection timeout

When an RPC connection attempt with bitcoind times out, include which ignored errors occurred in the exception message.

May provide clues of what has gone wrong.

test/functional/test_framework/test_node.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# Copyright (c) 2017-2022 The Bitcoin Core developers
+# Copyright (c) 2017-present The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Class for bitcoind node under test"""
@@ -265,6 +265,8 @@ class TestNode():
         """Sets up an RPC connection to the bitcoind process. Returns False if unable to connect."""
         # Poll at a rate of four times per second
         poll_per_s = 4
+        suppressed_errors = collections.defaultdict(int)
+        latest_error = ""
         for _ in range(poll_per_s * self.rpc_timeout):
             if self.process.poll() is not None:
                 # Attach abrupt shutdown error/s to the exception message
@@ -317,6 +319,8 @@ class TestNode():
                 # -342 Service unavailable, could be starting up or shutting down
                 if e.error['code'] not in [-28, -342]:
                     raise  # unknown JSON RPC exception
+                suppressed_errors[f"JSONRPCException {e.error['code']}"] += 1
+                latest_error = repr(e)
             except OSError as e:
                 # Suppress similarly to the above JSONRPCException errors.
                 if e.errno not in [
@@ -326,12 +330,16 @@ class TestNode():
                     errno.ECONNREFUSED  # Port not yet open?
                 ]:
                     raise  # unknown OS error
+                suppressed_errors[f"OSError {errno.errorcode[e.errno]}"] += 1
+                latest_error = repr(e)
             except ValueError as e:
                 # Suppress if cookie file isn't generated yet and no rpcuser or rpcpassword; bitcoind may be starting.
                 if "No RPC credentials" not in str(e):
                     raise
+                suppressed_errors["missing_credentials"] += 1
+                latest_error = repr(e)
             time.sleep(1.0 / poll_per_s)
-        self._raise_assertion_error("Unable to connect to bitcoind after {}s".format(self.rpc_timeout))
+        self._raise_assertion_error(f"Unable to connect to bitcoind after {self.rpc_timeout}s (ignored errors: {str(dict(suppressed_errors))}, latest error: {latest_error})")
 
     def wait_for_cookie_credentials(self):
         """Ensures auth cookie credentials can be read, e.g. for testing CLI with -rpcwait before RPC connection is up."""