Improve network config validation errors to show which fields are invalid.

Previously, uploading invalid config results in a generic error message
which makes it difficult to figure out what is wrong with the config

Change-Id: I307d2fc0669679b067389c722556eef3aae098b9
diff --git a/core/api/src/main/java/org/onosproject/net/config/InvalidFieldException.java b/core/api/src/main/java/org/onosproject/net/config/InvalidFieldException.java
new file mode 100644
index 0000000..f542dae
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/net/config/InvalidFieldException.java
@@ -0,0 +1,73 @@
+/*
+ * Copyright 2016-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.onosproject.net.config;
+
+/**
+ * Indicates a field of a configuration was invalid.
+ */
+public class InvalidFieldException extends RuntimeException {
+
+    private final String field;
+    private final String reason;
+
+    /**
+     * Creates a new invalid field exception about a given field.
+     *
+     * @param field field name
+     * @param reason reason the field is invalid
+     */
+    public InvalidFieldException(String field, String reason) {
+        super(message(field, reason));
+        this.field = field;
+        this.reason = reason;
+    }
+
+    /**
+     * Creates a new invalid field exception about a given field.
+     *
+     * @param field field name
+     * @param cause throwable that occurred while trying to validate field
+     */
+    public InvalidFieldException(String field, Throwable cause) {
+        super(message(field, cause.getMessage()));
+        this.field = field;
+        this.reason = cause.getMessage();
+    }
+
+    /**
+     * Returns the field name.
+     *
+     * @return field name
+     */
+    public String field() {
+        return field;
+    }
+
+    /**
+     * Returns the reason the field failed to validate.
+     *
+     * @return reason
+     */
+    public String reason() {
+        return reason;
+    }
+
+    private static String message(String field, String reason) {
+        return "Field \"" + field + "\" is invalid: " + reason;
+    }
+
+}