apport-retrace doesn't comment on bugs when add_gdb_info raises an IOError

Looking at the retracer's activity on bug 1639215, we can see that the retracer did something because it removed the need-amd64-retrace tag but we don't know what. It'd be helpful to know what happened during the retracing process rather than leaving people reading the bug in the dark.

Looking at the retracing log files we see the following:

ERROR: Invalid core dump: BFD: Warning: /tmp/apport_core_RVHov3 is truncated: expected core file size >= 2306977792, found: 2147479552.

Looking at add_gdb_info we see the following:

        if 'is truncated: expected core file size' in out:
        ...
           raise IOError(reason)

And apport-retrace does the following:

    try:
        report.add_gdb_info(sandbox)
    except IOError as e:
        apport.fatal(str(e))

Instead of just exiting, apport-retrace should comment on the bug about the core dump being corrupt and call crashdb.mark_retrace_failed().