Prints help on command-line syntax error

If argparse can't parse the command-line, or if UsageError is raised
from code, show the help message.
Prints help on "target directory doesn't exist"

People run into this a lot, for example because they give
"reprounzip x run" the rpz filename, like for "reprounzip x setup".
Prints help on "run is not a number"
Merge branch 'print-help-on-error' into 'master'