Bad error message when adding user before initializing

[chiptopher]

Bug report

When attempting to guet add without having done guet init, it prints a stack trace of the saying the database cannot be accessed.

Actual behavior?

(venv) ➜  ~ guet add us "name" email@localhost
Traceback (most recent call last):
  File "/home/user/workspace/guet/venv/bin/guet", line 11, in <module>
    load_entry_point('guet==1.0.0', 'console_scripts', 'guet')()
  File "/home/user/workspace/guet/venv/lib/python3.6/site-packages/guet-1.0.0-py3.6.egg/guet/main.py", line 22, in main
  File "/home/user/workspace/guet/venv/lib/python3.6/site-packages/guet-1.0.0-py3.6.egg/guet/executor.py", line 26, in execute
  File "/home/user/workspace/guet/venv/lib/python3.6/site-packages/guet-1.0.0-py3.6.egg/guet/commands/addcommitter.py", line 42, in execute
  File "/home/user/workspace/guet/venv/lib/python3.6/site-packages/guet-1.0.0-py3.6.egg/guet/gateway.py", line 108, in add_user
  File "/home/user/workspace/guet/venv/lib/python3.6/site-packages/guet-1.0.0-py3.6.egg/guet/gateway.py", line 121, in get_user
sqlite3.OperationalError: unable to open database file

Expected behavior?

An error message saying something to the effect of:
You must initialize guet before adding committers!

[chiptopher]

Same as #16