Making an error message for running decompress without arguments

[GabrielSimonetto]

When decompress used to be the default command, running ouch would prompt up the help menu, as it also does now, however, since we made the decompress command explicit, now it needs a default behaviour to throw an error message

Right now it panics:

$ ouch decompress
thread 'main' panicked at 'not yet implemented: Complain that no decompression arguments were given.', src/cli.rs:128:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[vrmiguel]

Closed by 4041084