efivar: handle error when reading --fromfile

The result of read() was stored in an unsigned variable, so the
test for a negative value would never work.

While I'm here, print a better error message for an empty file,
omitting the misleading errno message.

Reported by: Coverity
MFC after: 1 week
Sponsored by: Dell EMC Isilon

(cherry picked from commit d0f4e583bff67e2bdd816549c460da6daa3f67b1)