Add error check for invalid config

[ghost]

In the current version, the error message states TypeError: 'NoneType' object is not iterable when the 'rules' in the config file is not a dict (gh-274.) It would be helpful to improve the error message.

Additionally, it is for consistency, as other elements(ignore, locale, etc.) have similar error checks.

[coveralls]

coverage: 99.815%. remained the same
when pulling a941fb4 on koyuki7w:patch-1
into 2d10aaa on adrienverge:master.

[adrienverge]

Hello Yuki,

Thanks for this addition, this is a good idea.

I wonder whether "dict" is the best user-facing word (in "rules should be a dict"), because "dict" seems a Python-specific word to me. But anyway, we can always change it later if needed.
EDIT: We already use "dict" a few lines above (in "invalid config: not a dict") so I guess it's OK.

[ghost]

According to the YAML specification, "mapping" seems to be a more appropriate term. The current yamllint documentation uses "mapping" and does not mention "dict."

Perhaps all three instances of "dict" in the error message should be changed to "mapping."

yamllint/yamllint/config.py

Line 82 in 79a6b2b

raise YamlLintConfigError('invalid config: not a dict')

yamllint/yamllint/config.py

Line 86 in 79a6b2b

raise YamlLintConfigError('invalid config: rules should be a dict')

yamllint/yamllint/config.py

Lines 237 to 238 in 79a6b2b

	f'invalid config: rule "{rule.ID}": should be either "enable", '
	f'"disable" or a dict')

[adrienverge]

Thanks for your input @koyuki7w. I agree and I created #751 to address this.