doesn't seem to be handling non-existent commands

[dmalan]

doesn't seem to be handling non-existent commands (e.g., help50 foo where foo is not a command)?

$ help50 foo
Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.9/bin/help50", line 126, in <module>
    main()
  File "/home/travis/virtualenv/python2.7.9/bin/help50", line 43, in main
    proc = ptyprocess.PtyProcess.spawn(sys.argv[1:])
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/ptyprocess/ptyprocess.py", line 208, in spawn
    'executable: %s.' % command)
ptyprocess.ptyprocess.FileNotFoundError: The command was not found or was not executable: foo.

CC @kzidane

[dmalan]

@crossroads1112 expected behavior:

[cmlsharp]

Addressed in #254