Merge bitcoin/bitcoin#33960: log: Use more severe log level (warn/err) where appropriate

fa45a1503e log: Use LogWarning for non-critical logs (MarcoFalke)
fa0018d011 log: Use LogError for fatal errors (MarcoFalke)
22229de728 doc: Fix typo in init log (MarcoFalke)

Pull request description:

  Logging supports severity levels above info via the legacy `LogPrintf`. So use the more appropriate `LogError` or `LogWarning`, where it applies.

  This has a few small benefits:

  * It often allows to remove the manual and literal "error: ", "Warning:", ... prefixes. Instead the uniform log level formatting is used.
  * It is easier to grep or glance for more severe logs, which indicate some kind of alert.
  * `LogPrintf` didn't indicate any severity level, but it is an alias for `LogInfo`. So having the log level explicitly spelled out makes it easier to read the code.
  * Also, remove the redundant trailing `\n` newline, while touching.
  * Also, remove the `__func__` formatting in the log string, which is redundant with `-logsourcelocations`. Instead, use a unique log string for each location.

ACKs for top commit:
  l0rinc:
    Code review ACK fa45a1503e
  stickies-v:
    ACK fa45a1503e
  rkrux:
    crACK fa45a1503e

Tree-SHA512: 516d439c36716f969c6e82d00bcda03c92c8765a9e41593b90052c86f8fa3a3dacbb2c3dc98bfc862cefa54cae34842b488671a20dd86cf1d15fb94aa5563406

test/functional/feature_config_args.py

@@ -129,7 +129,7 @@ class ConfArgsTest(BitcoinTestFramework):
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf:
             conf.write('reindex=1\n')
 
-        with self.nodes[0].assert_debug_log(expected_msgs=['Warning: reindex=1 is set in the configuration file, which will significantly slow down startup. Consider removing or commenting out this option for better performance, unless there is currently a condition which makes rebuilding the indexes necessary']):
+        with self.nodes[0].assert_debug_log(expected_msgs=["[warning] reindex=1 is set in the configuration file, which will significantly slow down startup. Consider removing or commenting out this option for better performance, unless there is currently a condition which makes rebuilding the indexes necessary"]):
             self.start_node(0)
         self.stop_node(0)
 
@@ -229,7 +229,7 @@ class ConfArgsTest(BitcoinTestFramework):
                 )
 
     def test_log_buffer(self):
-        with self.nodes[0].assert_debug_log(expected_msgs=['Warning: parsed potentially confusing double-negative -listen=0\n']):
+        with self.nodes[0].assert_debug_log(expected_msgs=["[warning] Parsed potentially confusing double-negative -listen=0\n"]):
             self.start_node(0, extra_args=['-nolisten=0'])
         self.stop_node(0)