fix: 🐛 make postcss config error explicit

✅ Closes: #298

Author: kaisermann

Diff for: src/transformers/postcss.ts

@@ -33,7 +33,7 @@ async function process({
 
 async function getConfigFromFile(
   options: Options.Postcss,
-): Promise<Options.Postcss | null> {
+): Promise<{ config: Options.Postcss | null; error: string | null }> {
   try {
     /** If not, look for a postcss config file */
     const { default: postcssLoadConfig } = await import(`postcss-load-config`);
@@ -43,12 +43,18 @@ async function getConfigFromFile(
     );
 
     return {
-      plugins: loadedConfig.plugins,
-      // `postcss-load-config` puts all other props in a `options` object
-      ...loadedConfig.options,
+      error: null,
+      config: {
+        plugins: loadedConfig.plugins,
+        // `postcss-load-config` puts all other props in a `options` object
+        ...loadedConfig.options,
+      },
     };
   } catch (e) {
-    return null;
+    return {
+      config: null,
+      error: e,
+    };
   }
 }
 
@@ -59,20 +65,20 @@ const transformer: Transformer<Options.Postcss> = async ({
   options = {},
   map,
 }) => {
-  let fileConfig: Options.Postcss;
+  let fileConfig: { config: Options.Postcss; error: string };
 
   if (!options.plugins) {
     fileConfig = await getConfigFromFile(options);
-    options = { ...options, ...fileConfig };
+    options = { ...options, ...fileConfig.config };
   }
 
   if (options.plugins || options.syntax || options.parser) {
     return process({ options, content, filename, sourceMap: map });
   }
 
-  if (fileConfig === null) {
+  if (fileConfig.error !== null) {
     console.error(
-      `[svelte-preprocess] PostCSS configuration was not passed. If you expect to load it from a file make sure to install "postcss-load-config" and try again.`,
+      `[svelte-preprocess] PostCSS configuration was not passed or is invalid. If you expect to load it from a file make sure to install "postcss-load-config" and try again.\n\n${fileConfig.error}`,
     );
   }