nmbug-status: Clarify errors for illegible configs

Carl Worth pointed out that errors like:

  $ ./nmbug-status
  fatal: Not a git repository: '/home/cworth/.nmbug'
  fatal: Not a git repository: '/home/cworth/.nmbug'
  Traceback (most recent call last):
    File "./nmbug-status", line 254, in <module>
      config = read_config(path=args.config)
    File "./nmbug-status", line 73, in read_config
      return json.load(fp)
    File "/usr/lib/python2.7/json/__init__.py", line 290, in load
      **kw)
    File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
      return _default_decoder.decode(s)
    File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
      obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
      raise ValueError("No JSON object could be decoded")
  ValueError: No JSON object could be decoded

are not particularly clear.  With this commit, we'll get output like:

  $ ./nmbug-status
  fatal: Not a git repository: '/home/wking/.nmbug'
  No local branch 'config' in /home/wking/.nmbug.  Checkout a local
  config branch or explicitly set --config.

which is much more accessible.  I've also added user-friendly messages
for a number of other config-parsing errors.

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