fix(preprocessor): Improve handling of failed preprocessors

Closes karma-runner#1521

lib/preprocessor.js

@@ -27,8 +27,10 @@ var createPreprocessor = function (config, basePath, injector) {
       return
     }
 
+    var p
+
     try {
-      instances[name] = injector.get('preprocessor:' + name)
+      p = injector.get('preprocessor:' + name)
     } catch (e) {
       if (e.message.indexOf('No provider for "preprocessor:' + name + '"') !== -1) {
         log.warn('Can not load "%s", it is not registered!\n  ' +
@@ -39,6 +41,8 @@ var createPreprocessor = function (config, basePath, injector) {
 
       alreadyDisplayedWarnings[name] = true
     }
+
+    return p
   }
 
   patterns.forEach(function (pattern) {
@@ -80,10 +84,21 @@ var createPreprocessor = function (config, basePath, injector) {
             config[patterns[i]].join(', '), file.originalPath)
         } else {
           config[patterns[i]].forEach(function (name) {
-            if (!instances[name]) {
-              instantiatePreprocessor(name)
+            var p = instances[name]
+            if (p == null) {
+              p = instantiatePreprocessor(name)
             }
-            preprocessors.push(instances[name])
+
+            if (p == null) {
+              if (!alreadyDisplayedWarnings[name]) {
+                alreadyDisplayedWarnings[name] = true
+                log.warn('Failed to instantiate preprocessor %s', name)
+              }
+              return
+            }
+
+            instances[name] = p
+            preprocessors.push(p)
           })
         }
       }