ZipRequest: Fix decoding problems

Reading the bytes from the zip arcive inserted null characters and
 duplicate information. We can fix this by specifying the encoding
 with the reader before trying to read from it. Also checked that
 Gerrit does write in UTF-8 encoding.
VersionProcessor: Add aditional error checking to response - we may not
 get any response data if there are network issues.

Change-Id: I54c20ffea223523c43b34efa082e7484bb075dc0