oculus doesn't fail gracefully on Linux

[Greg Couch]

Command: oculus on

Traceback (most recent call last):
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/cmd_line/gui.py", line 121, in on_enter
cmd.execute()
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/core/commands/cli.py", line 1714, in execute
results.append(ci.function(session, **kw_args))
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/test/test.py", line 99, in run_commands
run(session, c)
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/core/commands/run.py", line 22, in run
return command.execute(log = log)
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/core/commands/cli.py", line 1714, in execute
results.append(ci.function(session, **kw_args))
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/core/devices/oculus/oculuscmd.py", line 20, in oculus
start_oculus(session)
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/core/devices/oculus/oculuscmd.py", line 46, in start_oculus
oc = Oculus_Rift(session)
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/core/devices/oculus/track.py", line 16, in __init__
self.connected = self.connect()
File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.5/site-packages/chimera/core/devices/oculus/track.py", line 19, in connect
from . import _oculus
ImportError: cannot import name '_oculus'

[Greg Couch]

Found by running test command.

[Tom Goddard]

Checked in a fix, but I don't have a linux system to test on.