Error when using unsupported PostCSS version (#361)

[thinkverse]

Added the postcss version check @ai talked about. Changed the wording a bit and used the built-in error function.

As for testing this functionality, not going to lie. I couldn't get this error to throw ones, even did the steps mentioned in the issue (#361) by using npm link to this version and installing plugins that use PostCSS 7 and it builds the CSS file and didn't throw a single error.

Seems like this error might be thrown very rarely? But I do hope it helps in the long run. 🙂

[coveralls]

Coverage remained the same at 81.41% when pulling e140a29 on thinkverse:unsupported-postcss-version-error into 4b9b66d on postcss:master.

[coveralls]

Coverage remained the same at 81.41% when pulling e140a29 on thinkverse:unsupported-postcss-version-error into 4b9b66d on postcss:master.

[ai]

@RyanZim this PR is ready to merge. My yarn/npm error message suggestion is not a blocker to merge and release it (especially since a lot of users have this problem right now).

[thinkverse]

I tried to find a good solution to the auto-discovery of the package manager but to no avail. Could keep looking for a solution for a later PR though of the feature is really needed.

[RyanZim]

KISS; no need for any fancy auto-detection. LGTM