init: improve corrupted/empty settings file error msg

The preceding "Unable to parse settings file" message lacked
the necessary detail and guidance for users on what steps to
take next in order to resolve the startup error.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

test/functional/feature_settings.py

@@ -53,7 +53,7 @@ class SettingsTest(BitcoinTestFramework):
         # Test invalid json
         with settings.open("w") as fp:
             fp.write("invalid json")
-        node.assert_start_raises_init_error(expected_msg='Unable to parse settings file', match=ErrorMatch.PARTIAL_REGEX)
+        node.assert_start_raises_init_error(expected_msg='does not contain valid JSON. This is probably caused by disk corruption or a crash', match=ErrorMatch.PARTIAL_REGEX)
 
         # Test invalid json object
         with settings.open("w") as fp: