Better log output for non-existing toc folders

[katywings]

The error when there is an non existing folder item in the toc is not very helpful:

(node:6332) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): TypeError: Cannot set property 'href' of undefined
(node:6332) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Reproduction (This config should do it):

toc: [
  { path: 'i_am_not_existing', children: [
    { path: 'child_in_i_am_not_existing' }
  ] }
]