src.ctf.fs: append error causes in ctf_fs_file_open

src.ctf.fs: append error causes in ctf_fs_file_open

I hit an fopen failure in ctf_fs_file_open and noticed we were missing
some error causes for the message to be precise.  This patch adds them.

This is the result I now get when fopen fails:

    ...
    CAUSED BY [auto-disc-source-ctf-fs-37: 'source.ctf.fs'] (/home/smarchi/src/babeltrace/src/plugins/ctf/fs-src/file.c:98)
      Cannot open file: Too many open files: path=/tmp/lttng-traces/000300/mon_10.196.130.21-20180202-144403/ust/uid/0/64-bit/ch-1_4, mode=rb

Change-Id: I4dae278695b56cbedc9c7e3e5a67bc60fe6584d7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2230
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>