check HTTP response code in downloadFile
fixed a trouble in UpdateService:
  - put a corrupted zip in /sdcard/release.zip
  - run dSploit
  - dSploit detect that msf are available as local archive
  - update MSF
  - corrupted file found, the archive be removed.
  - exit from dSploit
  - run it again

there happens the issue:
 UpdateService will fetch archive metadata only if mMsfInfo.version is null.
 but that field has been initialized when the local file was found.
 so the running UpdateService instance will have a corrupt ArchiveMetadata,
 with `mMsfInfo.url = null`.

this leads to problem, because the archive does not exists locally
and cannot be downloaded due to missing url.

thanks to @megahz for the report.