Here are the error lines from /var/log/Xorg.0.log:
[    77.362] (EE) dbus-core: error connecting to system bus: org.freedesktop.DBus.Error.FileNotFound (Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directory)
[    77.376] (EE) open /dev/dri/card0: No such file or directory
[    77.376] (EE) open /dev/dri/card0: No such file or directory
[    77.376] (EE) open /dev/dri/card0: No such file or directory
[    77.376] (EE) Screen 0 deleted because of no matching config section.
[    77.376] (EE) Device(s) detected, but none match those in the config file.
[    77.377] (EE) open /dev/dri/card0: No such file or directory
[    77.377] (EE) open /dev/dri/card0: No such file or directory
[    77.377] (EE) open /dev/dri/card0: No such file or directory
[    77.377] (EE) Screen 0 deleted because of no matching config section.
[    77.377] (EE) Device(s) detected, but none match those in the config file.
[    77.377] (EE)
[    77.377] (EE) no screens found(EE)

/dev/dri/card0 is present and readable:
ls /dev/dri
total 0
drwxr-xr-x  2 root root       120 May 21 20:36 by-path/
crw-rw----+ 1 root video 226,   0 May 21 20:36 card0
crw-rw----+ 1 root video 226,   1 May 21 20:36 card1
crw-rw----+ 1 root video 226, 128 May 21 20:36 renderD128
crw-rw----+ 1 root video 226, 129 May 21 20:36 renderD129

egrep -i 'VGA|NVIDIA' ~/tmp/sysinfo.txt
00:02.0 VGA compatible controller: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller (rev 06)
01:00.0 3D controller: NVIDIA Corporation GK104M [GeForce GTX 870M] (rev a1)

/proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-4.15.0-22-generic root=UUID=d9065d6f-7fee-451b-a9df-5a6863a4b1e3 ro quiet splash vt.handoff=1