Fix: add error handling when custom.ord.json doesn't exist

[zongqichen]

If there is no custom.ord.json filed existed, there will be throw an error: Cannot find module...., we need to reflect more clear error message.

[Fannon]

You mean if you configured it in .cdsrc but the link to the file does not exist?

Yes, would be good to point that out, also stating the name of the .cdsrc option and the value / path that cannot be found.