Fixes a bug in appending part file with a URL

-[NSFileManager attributesOfItemAtPath:error:] may fail and the error wasn't
being handled. The body content length would be 0 if that is the case, and this
would result in the content length being incorrect. Let's just throw the error
back instead of silently handling this issue.
Merge pull request #1629 from kylef/bug/appendpartwithfile

Fixes a bug in appending part file with a URL