Handle JSON errors in the config

[alichtman]

Traceback (most recent call last):
  File "/usr/local/bin/shallow-backup", line 11, in <module>
    load_entry_point('shallow-backup==2.6', 'console_scripts', 'shallow-backup')()
    └ <function load_entry_point at 0x10cd8de60>
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
           │          │       └ {}
           │          └ ()
           └ <click.core.Command object at 0x10c776250>
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
         │           └ <click.core.Context object at 0x10d293c10>
         └ <click.core.Command object at 0x10c776250>
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           │          │                └ <click.core.Context object at 0x10d293c10>
           │          └ <click.core.Command object at 0x10c776250>
           └ <click.core.Context object at 0x10d293c10>
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
           │         │       └ {'no_splash': True, 'dotfiles': True, 'separate_dotfiles_repo': True, 'all': False, 'configs': False, 'delete_config': False, 'd...
           │         └ ()
           └ <function cli at 0x10d312ef0>
  File "/usr/local/lib/python3.7/site-packages/shallow_backup-2.6-py3.7.egg/shallow_backup/__main__.py", line 69, in cli
    backup_config = get_config()
                    └ <function get_config at 0x10d071680>
  File "/usr/local/lib/python3.7/site-packages/shallow_backup-2.6-py3.7.egg/shallow_backup/config.py", line 18, in get_config
    config = json.load(f)
             │         └ <_io.TextIOWrapper name='/Users/alichtman/.shallow-backup' mode='r' encoding='UTF-8'>
             └ <module 'json' from '/usr/local/Cellar/python/3.7.4_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py'>
  File "/usr/local/Cellar/python/3.7.4_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py", line 296, in load
    parse_constant=parse_constant, object_pairs_hook=object_pairs_hook, **kw)
  File "/usr/local/Cellar/python/3.7.4_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/__init__.py", line 348, in loads
    return _default_decoder.decode(s)
           │                       └ '{\n    "backup_path": "~/shallow-backup",\n    "dotfiles": [\n        ".aliases",\n        ".ctags",\n        ".config/pycodest...
           └ <json.decoder.JSONDecoder object at 0x10d04d910>
  File "/usr/local/Cellar/python/3.7.4_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               │               │      │  └ '{\n    "backup_path": "~/shallow-backup",\n    "dotfiles": [\n        ".aliases",\n        ".ctags",\n        ".config/pycodest...
               │               │      └ <built-in method match of re.Pattern object at 0x10d015570>
               │               └ '{\n    "backup_path": "~/shallow-backup",\n    "dotfiles": [\n        ".aliases",\n        ".ctags",\n        ".config/pycodest...
               └ <json.decoder.JSONDecoder object at 0x10d04d910>
  File "/usr/local/Cellar/python/3.7.4_1/Frameworks/Python.framework/Versions/3.7/lib/python3.7/json/decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)
               │              │  └ 0
               │              └ '{\n    "backup_path": "~/shallow-backup",\n    "dotfiles": [\n        ".aliases",\n        ".ctags",\n        ".config/pycodest...
               └ <json.decoder.JSONDecoder object at 0x10d04d910>
json.decoder.JSONDecodeError: Expecting ',' delimiter: line 24 column 9 (char 483)