ceph_authtool.cc: catch ceph::buffer::end_of_buffer exception

Fix for:

CID 717142 (#1 of 1): Uncaught exception (UNCAUGHT_EXCEPT)
 root_function: In function main(int, char const **) an exception
 of type ceph::buffer::end_of_buffer is thrown and never caught.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
ceph_authtool.cc: catch ceph::FailedAssertion exception

Fix for:

CID 717141 (#1-19): Uncaught exception (UNCAUGHT_EXCEPT)
 root_function: In function main(int, char const **) an exception
 of type ceph::FailedAssertion is thrown and never caught.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>