errors: add missing ERR_ prefix on util.callbackify error

The `FALSY_VALUE_REJECTION` error code added by
https://github.com/nodejs/node/pull/12712 did not have the `ERR_` prefix,
nor was it added to the errors.md documentation. Add the prefix in for
consistency.

PR-URL: https://github.com/nodejs/node/pull/13604
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/internal/errors.js

@@ -115,6 +115,7 @@ E('ERR_ASSERTION', (msg) => msg);
 E('ERR_CONSOLE_WRITABLE_STREAM',
   (name) => `Console expects a writable stream instance for ${name}`);
 E('ERR_CPU_USAGE', (errMsg) => `Unable to obtain cpu usage ${errMsg}`);
+E('ERR_FALSY_VALUE_REJECTION', 'Promise was rejected with falsy value');
 E('ERR_HTTP_HEADERS_SENT',
   'Cannot render headers after they are sent to the client');
 E('ERR_HTTP_INVALID_CHAR', 'Invalid character in statusMessage.');
@@ -166,7 +167,6 @@ E('ERR_SOCKET_BAD_PORT', 'Port should be > 0 and < 65536');
 E('ERR_SOCKET_DGRAM_NOT_RUNNING', 'Not running');
 E('ERR_V8BREAKITERATOR', 'full ICU data not installed. ' +
   'See https://github.com/nodejs/node/wiki/Intl');
-E('FALSY_VALUE_REJECTION', 'Promise was rejected with falsy value');
 // Add new errors from here...
 
 function invalidArgType(name, expected, actual) {