tools: quagga-reload should raise Exception instead of exiting

tools: quagga-reload should raise Exception instead of exiting

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
NCLU imports quagga-reload.py and uses its Config class to parse
Quagga.conf.  The Config class will call 'vtysh -m -f Quagga.conf" and
if that exited with an error Config would call sys.exit(1) which in my
cases causes the NCLU daemon to exit which is bad.  The fix is to have
the Config class raise an exception instead of exiting, then NCLU can
catch the exception, log it and move on.

(cherry picked from commit 276887bb1c2961fa37b42ce7160346f1417577a8)