Restrict the types of errors that return file_exists.

I have seen some 'file exists' error messages in the log files.  We have to make
sure these are really race conditions and not something else.
Use HTTP 1.0 instead of 1.1 for ddfs operations.

We have seen an issue in ddfs_http:http_put_conn where the connect
succeeds and returns a Socket, then sending the Head with gen_tcp:send
succeeds but the call to gen_tcp:send in send_body fails with
{badmatch,{error,closed}}.
This might be an HTTP/1.1 issue.
Do not assume scheduling issue when the job has finished.

If schedule is called when the job has no tasks left, it should gracefully
return without printing scary warnings.
Merge pull request #406 from pooya/shayan/errors

Shayan/errors