Throw error if loadConfig cannot find file by path

[henrahmagix]

If given a path to a file that doesn't exist, sass-lint should throw an
error to let the user know it cannot find the config file. Otherwise the
user will have no idea that the default config is being used and theirs
is being ignored.

When not given a config path, continue as usual: return an empty config.

This also fixes relative path issues in the failures test suite.

---

What do the changes you have made achieve?

They let the user know if the given path to a config file does not exist instead of silently returning an empty config.

Are there any new warning messages?

Yes: 1 new Error thrown.

> confHelpers.loadConfig('notexist.yml')
Error: Cannot find configPath notexist.yml
    at [stack trace...]

Have you written tests?

Yes, and fixed the config file paths in tests/failures.js.

Have you included relevant documentation

No: I think the error thrown should be the natural result of giving an incorrect filepath, rather than a "new feature".

Which issues does this resolve?

Fixes #1006

<DCO 1.1 Signed-off-by: Henry Blyth <[email protected]>

[coveralls]

Coverage increased (+0.001%) to 97.519% when pulling 45a2214 on henrahmagix:fail-for-config-not-found into d96ed96 on sasstools:develop.

[DanPurdy]

Thanks for this PR, I'm not sure if the unhandled error throwing is really the way to go at the moment, let me run this and have a look at it.

[henrahmagix]

Hi @DanPurdy,

Thanks for taking a look. What do you think now?

Cheers,
Henry