src.ctf.fs: error out when failing to create index

src.ctf.fs: error out when failing to create index

This patch makes src.ctf.fs fail if it is not able to build an index for
a data stream file.  This is generally a sign of a corrupted trace.

The plan is to be strict by default (abort if we find the trace looks
corrupted), but to later introduce a parameter to tell the component to
make those checks not fatal.

So for now, we can assume that the index will always be present, hence
the comment change in fs.h.

Without this patch, the trace provided as a test leads to this abort:

    10-17 15:59:00.749 14522 14522 W PLUGIN/SRC.CTF.FS/DS build_index_from_idx_file@data-stream-file.c:462 [auto-disc-source-ctf-fs] Invalid LTTng trace index file; indexed size != stream file size: file-size=16699392, total-packets-size=16896000
    10-17 15:59:00.749 14522 14522 W PLUGIN/SRC.CTF.FS/DS build_index_from_stream_file@data-stream-file.c:596 [auto-disc-source-ctf-fs] Invalid packet size reported in file: stream="/tmp/single/ch-1_17", packet-offset=12701696, packet-size-bytes=4194304, file-size=16699392
    10-17 15:59:00.749 14522 14522 W PLUGIN/SRC.CTF.FS add_ds_file_to_ds_file_group@fs.c:789 [auto-disc-source-ctf-fs] Failed to index CTF stream file '/tmp/single/ch-1_17'

     (╯°□°)╯︵ ┻━┻  /home/smarchi/src/babeltrace/src/plugins/ctf/fs-src/fs.c:1572: fix_index_lttng_event_after_packet_bug(): Assertion `index` failed.
    [1]    14522 abort      ./src/cli/babeltrace2 /tmp/single

The reason is that the index fails to build (the cumulative packet sizes
is larger than the data stream file size), but we keep going anyway.
The fix_index_lttng_event_after_packet_bug function, expecting an index
to always be present, hits that assert.

I added the problematic trace to our test trace collection, and a new
test script "test_fail" to test how the src.ctf.fs component class
handles invalid traces.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I5320102352eb8ef5004f0555dec2cc2736297a4f