stream_file: print strerror() when failing to open a file

stream_file: print strerror() when failing to open a file

stream_file always printed "File not found" if it could not open a
file, even though this could be due to other reasons such as
permission problems. Print strerror() information instead. This
changes the output for "mplayer /etc/shadow" from
File not found: '/etc/shadow'
to
Cannot open file '/etc/shadow': Permission denied