Propagate Errors in DefaultHttpRequestBuilder

This commit introduces the new `WebClientException` for wrapping
client-side exceptions.
This exception is now thrown when now message encoder can be found for
writing the request body as an HTTP request message.

Fixes #83

spring-web-reactive/src/main/java/org/springframework/web/client/reactive/DefaultHttpRequestBuilder.java

@@ -170,10 +170,9 @@ public class DefaultHttpRequestBuilder implements HttpRequestBuilder {
 								mediaType));
 			}
 			else {
-				// TODO: wrap with client exception?
-				request.setBody(Flux.error(new IllegalStateException("Can't write request body" +
+				throw new WebClientException("Can't write request body " +
 						"of type '" + requestBodyType.toString() +
-						"' for content-type '" + mediaType.toString() + "'")));
+						"' for content-type '" + mediaType.toString() + "'");
 			}
 		}
 

spring-web-reactive/src/main/java/org/springframework/web/client/reactive/WebClientException.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2002-2016 the original author or authors.
+ *
+ * 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.springframework.web.client.reactive;
+
+import org.springframework.core.NestedRuntimeException;
+
+/**
+ * Base class for exceptions thrown by {@link WebClient} whenever
+ * it encounters client-side errors.
+ *
+ * @author Brian Clozel
+ */
+public class WebClientException extends NestedRuntimeException {
+
+	public WebClientException(String msg) {
+		super(msg);
+	}
+
+	public WebClientException(String msg, Throwable cause) {
+		super(msg, cause);
+	}
+}