beesh: declare variable 'nocheck' as a local variable

So every auxiliary variable in the function fetch_one_file is now
declared as 'local'.
beesh: check whether the fetched file has a size greater than zero

If wget created a file which has a size of 0 we know that wget was not
successfull. So we can throw an error, delete the file and exit.
beesh: fetch_one_file: quote argument from rm