Lower 'no cbmc-viewer.json' logging from error to info

Viewer currently logs an error if the configuration file
cbmc-viewer.json is missing.  Viewer does not depend on the
configuration file, it is merely guided to better results by the
configuration file.  This change lowers the error message to an info
message (that is not displayed without the --verbose or --debug
flags).

src/cbmc_viewer/configt.py

@@ -21,7 +21,7 @@ def __init__(self, config_file=None):
  return
 
  if not Path(config_file).exists():
- logging.error("Config file does not exist: %s", config_file)
+ logging.info("Config file does not exist: %s", config_file)
  return
 
  config_data = parse.parse_json_file(config_file)