improve support for export to stdout

When passing `-` as output file, it mostly works: Both curl and wget treats that as a redirection to stdout.

Unfortunately the shell script does not expect this (which is a common pattern in POSIX-like tools):

a) An annoying trailing line containing the lone dash is appended.
b) Exporting as slides does not support this.

For both cases, dash should here be captured and an appropriate action (either nothing or a graceful error message emitted to stderr) should be taken instead.