Do not call sys.exit in ligo.gracedb.rest module

The ligo.gracedb.rest module should raise exceptions instead of calling sys.exit() so that it is possible to inspect stack traces. Generally, methods in importable modules should not call sys.exit(), just as C library functions should not call exit().

This change turns output_and_die into a class method that can be overridden in the classes in the ligo.gracedb.cli module. However, ideally we should get rid of the output_and_die method altogether, raise ordinary exceptions in ligo.gracedb.rest, and handle user-friendly rendering of errro messages in ligo.gracedb.cli.