Improved text of exception message

When some third-party module has syntax error inside its composer.json, it's very hard to determine which
module caused an exception. 

This PR changes default message with the following:

```
Some_Module's composer.json error: Decoding failed: Syntax error
```

Author: vovayatsyuk

lib/internal/Magento/Framework/Module/PackageInfo.php

@@ -82,7 +82,15 @@ private function load()
             foreach ($this->componentRegistrar->getPaths(ComponentRegistrar::MODULE) as $moduleName => $moduleDir) {
                 $key = $moduleDir . '/composer.json';
                 if (isset($jsonData[$key]) && $jsonData[$key]) {
-                    $packageData = \Zend_Json::decode($jsonData[$key]);
+                    try {
+                        $packageData = \Zend_Json::decode($jsonData[$key]);
+                    } catch (\Zend_Json_Exception $e) {
+                        throw new \Zend_Json_Exception(
+                            sprintf("%s's composer.json error: ", $moduleName) .
+                            $e->getMessage()
+                        );
+                    }
+
                     if (isset($packageData['name'])) {
                         $this->packageModuleMap[$packageData['name']] = $moduleName;
                     }