Give message rather than error if not in a CARME directory.

[jkuruzovich]

Currently you get this ugly message if you run a carme command other than new and you aren't in a carme directory.

Traceback (most recent call last):
  File "/usr/local/bin/carme", line 11, in <module>
    sys.exit(carme())
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.5/dist-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)