call stop instead of quit

- add test to check that --help throws error

Calling `quit()` is not testable and may be too harsh and even conflict with CRAN policy:

> Compiled code should never terminate the R process within which it is running. Thus C/C++ calls to assert/abort/exit, Fortran calls to STOP and so on must be avoided. Nor may R code call q().

OTOH, `stop()` forcibly prints an error message which I have been unable to mute.

Fixes #4 (=includes it).