lib: restore err.errno to logs for symlink errors

This info (err.errno) is the only piece of information
in the error object that is not redundant to err.stack.

Author: DeeDeeG

lib/configure.js

@@ -101,12 +101,12 @@ function configure (gyp, argv, callback) {
     fs.unlink(symlinkDestination, function (err) {
       if (err && err.code !== 'ENOENT') {
         log.warn('python symlink', 'error when attempting to remove existing symlink')
-        log.warn('python symlink', err.stack)
+        log.warn('python symlink', err.stack, 'errno: ' + err.errno)
       }
       fs.symlink(python, symlinkDestination, function (err) {
         if (err) {
           log.warn('python symlink', 'error when attempting to create Python symlink')
-          log.warn('python symlink', err.stack)
+          log.warn('python symlink', err.stack, 'errno: ' + err.errno)
         }
       })
     })