Raise error when mbed_app.json is bad

tools/config.py

@@ -384,6 +384,7 @@ class Config(object):
         top_level_dirs may be None (in this case, the constructor will not
         search for a configuration file).
         """
+        config_errors = []
         app_config_location = app_config
         if app_config_location is None:
             for directory in top_level_dirs or []:
@@ -399,8 +400,10 @@ class Config(object):
             self.app_config_data = json_file_to_dict(app_config_location) \
                                    if app_config_location else {}
         except ValueError as exc:
-            sys.stderr.write(str(exc) + "\n")
             self.app_config_data = {}
+            config_errors.append(
+                ConfigException("Could not parse mbed app configuration from %s"
+                                % app_config_location))
 
         # Check the keys in the application configuration data
         unknown_keys = set(self.app_config_data.keys()) - \
@@ -433,7 +436,7 @@ class Config(object):
 
         self._process_config_and_overrides(self.app_config_data, {}, "app",
                                            "application")
-        self.config_errors = None
+        self.config_errors = config_errors
 
     def add_config_files(self, flist):
         """Add configuration files
@@ -806,6 +809,7 @@ class Config(object):
         """
         # Update configuration files until added features creates no changes
         prev_features = set()
+        self.validate_config()
         while True:
             # Add/update the configuration with any .json files found while
             # scanning