Clean-up: coverity warns of uncaught exception during logging

Coverity reports:

  1502349 Uncaught exception If the exception is ever thrown, the
  program will crash.

  In lttng::​file_descriptor::​~file_descriptor(): A C++ exception is
  thrown but never caught (CWE-248)

and

  1502348 Uncaught exception If the exception is ever thrown, the
  program will crash.

  In main: A C++ exception is thrown but never caught (CWE-248)

Both have the same cause: libfmt should not be used in "final" catch
blocks before returning to non exception-safe code in order to contain
all exceptions.

As we add custom formaters, it could be interesting to revisit this and
provide a noexcept wrapper for fmt::format. For the moment not much is
lost (beyond format string type safety) from using the existing logging
macros directly.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia70505517678ae182f6479feeb264c9402aa1381