Fix: lttng-destroy: string formating error when default session is unset

Fix: lttng-destroy: string formating error when default session is unset

Using `lttng destroy` when no default session is set in .lttngrc results
in the following print-out:

  Error: Can't find valid lttng config /root/lttng-build/home/.lttngrc
  Did you create a session? (lttng create <my_session>)
  Error: Failed to format string: string pointer is null

This is because the client attempts to format the following message:
  ERR_FMT("Session `{}` not found", spec.value);

When no default session could be found in .lttngrc, spec.value is left
at nullptr and it is assumed that the listing succeeded.

A new CLI-specific exception, no_default_session_error, is added to the
project and thrown when the session listing fails. This allows the
calling code to mark the listing as having failed.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I33b4f38a424f22dfa9d3628cf12441b59df53f12