When the path to "resolved_embedded_item" was shorter than
the path to the bundle being fixed up, fixup_bundle would
fail with a cmake error like:

  "string end index: 110 is out of range 0 - 85"

Detect when the path of resolved_embedded_item is too short
to be embedded in the bundle, and report the proper error
message, so the poor developer reading it has a snowball's
chance of actually fixing the issue.