sessiond: log error message when libkmod operations fail

sessiond: log error message when libkmod operations fail

Issue observed
--------------

When loading or unloading modules and libkmod is being used, there are
no details available in the `lttng-sessiond` error ouput.

For example,

```
$ sudo -E lttng-sessiond
Error: Unable to load required module lttng-ring-buffer-client-discard
Warning: No kernel tracer available
```

When libkmod is not available or disabled in the configuration
options, `lttng-sessiond` falls back to invoking `modprobe` via a
`system` call. The command's error output will be visible and provide
the necessary details, eg.

```
$ sudo -E lttng-sessiond
modprobe: FATAL: Module lttng-ring-buffer-client-discard not found in directory /usr/lib/modules/6.2.0-36-generic
Error: Unable to load required module lttng-ring-buffer-client-discard
Warning: No kernel tracer available
```

Solution
--------

Include the error message from `strerror` in the message that is
logged via the `DBG` or `ERR` macros.

The error is no clearer for users, eg.

```
$ sudo -E lttng-sessiond
PERROR - 12:00:05.004593045 [Main]: Unable to load required module
lttng-ring-buffer-client-discard: No such file or directory (in
modprobe_lttng() at modprobe.cpp:396)
Warning: No kernel tracer available
```

Known drawbacks
---------------

The debug message emitted when a non-obligatory kernel module fails to
load is now printed with `PERROR`.

Change-Id: Ibd25614a6c5b5dd3b801063eafc272a4017058cd
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>