Fix docker import tests

For me when I run the test I see:
```
Downloading from http://nourl/bad
Importing    283 B
Untar re-exec error: exit status 1: output: unexpected EOF
```
and nothing about "dial tcp" so it appears that the output is
system dependent and therefore we can't really check it. I think
checking for non-zero exit code is sufficient so I'm removing this
string check.

Signed-off-by: Doug Davis <dug@us.ibm.com>

chrootarchive/archive_unix.go

@@ -80,7 +80,7 @@ func invokeUnpack(decompressedArchive io.Reader, dest string, options *archive.T
 		// pending on write pipe forever
 		io.Copy(ioutil.Discard, decompressedArchive)
 
-		return fmt.Errorf("Untar re-exec error: %v: output: %s", err, output)
+		return fmt.Errorf("Error processing tar file(%v): %s", err, output)
 	}
 	return nil
 }