Looks like a minor oversight.
rbt=# set log_error_verbosity to terse;
SET
rbt=# set log_error_verbosity to default;
SET
rbt=# set log_error_verbosity to verbose;
ERROR: syntax error at or near "verbose" at character 28
LINE 1: set log_error_verbosity to verbose;
^
rbt=# set log_error_verbosity to 'terse';
SET
rbt=# set log_error_verbosity to 'verbose';
SET
rbt=# set log_error_verbosity to 'verboset';
ERROR: invalid value for parameter "log_error_verbosity": "verboset"