Avoid resolving promise with undefined; always throwing not found error.

[chriszarate]

The core promise can resolve to undefined since there is no fallback return / throw statement.

In the (very common) scenario where the import file is (1) correctly formed (ends with .css, etc.) but (2) does not exist, the promise will resolve to undefined instead of rejecting. This is especially problematic when walking through an array of possible paths where the import might be located, as in postcss-advanced-variables's importPaths directive:

https://github.com/jonathantneal/postcss-advanced-variables#importpaths

[chriszarate]

Project abandoned, no longer using this