Fix(cli): Show error message for invalid value
Fix(cli): Show error message for invalid value

For example, the command:
  `canonical_data_syncer --verbosity=quiett`

Now produces:
  Error: invalid value for '--verbosity': 'quiett'

And shows the help message.

Previously, the program silently ignored the invalid value, and operated
with the default value.

Fixes: #36

exercism/configlet