test: display abrupt shutdown errors in console output

Making it easier to debug errors in the CI environment,
particularly in scenarios where it's not immediately clear
what happened nor which node crashed (or shutdown abruptly).

test/functional/test_framework/test_node.py

@@ -232,8 +232,13 @@ class TestNode():
         poll_per_s = 4
         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
+                self.stderr.seek(0)
+                str_error = ''.join(line.decode('utf-8') for line in self.stderr)
+                str_error += "************************\n" if str_error else ''
+
                 raise FailedToStartError(self._node_msg(
-                    'bitcoind exited with status {} during initialization'.format(self.process.returncode)))
+                    f'bitcoind exited with status {self.process.returncode} during initialization. {str_error}'))
             try:
                 rpc = get_rpc_proxy(
                     rpc_url(self.datadir, self.index, self.chain, self.rpchost),