cephfs-shell: make onecmd() print proper error msg

cephfs-shell: make onecmd() print proper error msg

Rationale: Whenever a python exception occurred in cephfs-shell,
           it would often only be the exception message but doesn't
           say anything about the type of exception. For example if
           `ZeroDivisionError: division by zero` occurred, the onecmd()
           would print `division by zero` but will omit the type of
           exception. In this case it's easy to understand but let's
           say an `KeyError` exception occurred for a key `9999` which
           is not existent in the dictionary, onecmd() would print
           just `9999` in this scenario and it would be very difficult
           to interpret what type of error it is.

Fixes: https://tracker.ceph.com/issues/55536
Signed-off-by: Dhairya Parmar <dparmar@redhat.com>