Description of problem:
flatpak-coredumpctl is crashing (python exception) whenever gdb crashes. It needs to handle CalledProcessError here.

Version-Release number of selected component:
flatpak-1.4.2-2.fc30

Additional info:
reporter:       libreport-2.10.1
cmdline:        /usr/bin/python3 /usr/bin/flatpak-coredumpctl org.gnome.Epiphany.Devel//master
crash_function: check_call
exception_type: subprocess.CalledProcessError
executable:     /usr/bin/flatpak-coredumpctl
interpreter:    python3-3.7.3-3.fc30.x86_64
kernel:         5.1.17-300.fc30.x86_64
runlevel:       N 5
type:           Python3
uid:            1000

Truncated backtrace:
subprocess.py:347:check_call:subprocess.CalledProcessError: Command '['flatpak', 'run', '--filesystem=home', '--filesystem=/tmp', '--command=gdb', '--devel', 'org.gnome.Epiphany.Devel//master', '/usr/libexec/webkit2gtk-4.0/WebKitWebProcess', '/tmp/tmptrocbps8']' returned non-zero exit status 139.

Traceback (most recent call last):
  File "/usr/bin/flatpak-coredumpctl", line 83, in <module>
    coredumper.run()
  File "/usr/bin/flatpak-coredumpctl", line 58, in run
    subprocess.check_call(flatpak_command)
  File "/usr/lib64/python3.7/subprocess.py", line 347, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['flatpak', 'run', '--filesystem=home', '--filesystem=/tmp', '--command=gdb', '--devel', 'org.gnome.Epiphany.Devel//master', '/usr/libexec/webkit2gtk-4.0/WebKitWebProcess', '/tmp/tmptrocbps8']' returned non-zero exit status 139.

Local variables in innermost frame:
popenargs: (['flatpak', 'run', '--filesystem=home', '--filesystem=/tmp', '--command=gdb', '--devel', 'org.gnome.Epiphany.Devel//master', '/usr/libexec/webkit2gtk-4.0/WebKitWebProcess', '/tmp/tmptrocbps8'],)
kwargs: {}
retcode: 139
cmd: ['flatpak', 'run', '--filesystem=home', '--filesystem=/tmp', '--command=gdb', '--devel', 'org.gnome.Epiphany.Devel//master', '/usr/libexec/webkit2gtk-4.0/WebKitWebProcess', '/tmp/tmptrocbps8']