Friendlier error messages for two types of bad parameters.

lib/sw-precache.js

@@ -116,9 +116,29 @@ function generate(params, callback) {
     });
 
     Object.keys(params.dynamicUrlToDependencies).forEach(function(dynamicUrl) {
+      if (!Array.isArray(params.dynamicUrlToDependencies[dynamicUrl])) {
+        throw Error(util.format(
+          'The value for the dynamicUrlToDependencies.%s option must be an Array.', dynamicUrl));
+      }
+
       var filesAndSizesAndHashes = params.dynamicUrlToDependencies[dynamicUrl]
         .sort()
-        .map(getFileAndSizeAndHashForFile);
+        .map(function(file) {
+          try {
+            return getFileAndSizeAndHashForFile(file);
+          } catch(e) {
+            // Provide some additional information about the failure if the file is missing.
+            if (e.code === 'ENOENT') {
+              params.logger(util.format(
+                '%s was listed as a dependency for dynamic URL %s, but the file does not exist. ' +
+                'Either remove the entry as a dependency, or correct the path to the file.',
+                file, dynamicUrl
+              ));
+            }
+            // Re-throw the exception unconditionally, since this should be treated as fatal.
+            throw e;
+          }
+        });
       var concatenatedHashes = '';
 
       filesAndSizesAndHashes.forEach(function(fileAndSizeAndHash) {