EntityFile should throw an exception if the source file doesn't exist

Problem/Motivation

The EntityFile destination plugin copies files from one place to another. At the moment, it doesn't check if the source path actually exists, which means that copy() might eventually be called with an invalid source path. When it returns FALSE in that circumstance, the user will get a profoundly unhelpful "file could not be copied" exception.

Proposed Resolution

Before doing anything else, EntityFile should check if the source file actually exists, and throw a helpful exception if it doesn't.

Remaining Tasks

Discuss, then create a patch if needed.

API Changes

None.

UI Changes

None.