preprocessor errors and missing files are swallowed

[samsch]

Reproduction repo: https://github.com/samsch/css-modules-transform-scss-no-error

Preprocessor errors are swallowed and ignored. Specifically, errors thrown from the function exported from the preprocessCss option file don't cause the build to fail.

Additionally, if a require()d .scss file isn't found, the build continues without error.

Both cases are shown in the repro repo.