Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 28752)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -2530,13 +2530,16 @@
 		wp_send_json_error();
 	}
 
-	if ( ! current_user_can( 'read_post', $post->ID ) ) {
+	if ( empty( $_POST['shortcode'] ) || ! current_user_can( 'read_post', $post->ID ) ) {
 		wp_send_json_error();
 	}
 
 	setup_postdata( $post );
 
-	$parsed = $wp_embed->run_shortcode( $_POST['content'] );
+	// If the URL cannot be embedded, return eror message with call wp_send_json_error()
+	add_filter( 'embed_maybe_make_link', '_wpview_embed_error', 20, 2 );
+
+	$parsed = $wp_embed->run_shortcode( $_POST['shortcode'] );
 	$parsed = do_shortcode( $parsed );
 
 	wp_send_json_success( $parsed );
Index: src/wp-admin/includes/misc.php
===================================================================
--- src/wp-admin/includes/misc.php	(revision 28752)
+++ src/wp-admin/includes/misc.php	(working copy)
@@ -824,3 +824,15 @@
 }
 // Run later as we have to set DOING_AUTOSAVE for back-compat
 add_filter( 'heartbeat_received', 'heartbeat_autosave', 500, 2 );
+
+/**
+ * Send error message when an URL cannot be embedded. Used in wp_ajax_parse_embed().
+ *
+ * @access private
+ * @since 4.0
+ */
+function _wpview_embed_error( $output, $url ) {
+	wp_send_json_error( array(
+		'message' => sprintf( __( '%s failed to embed.' ), esc_url( $url ) ),
+	) );
+}
Index: src/wp-includes/js/mce-view.js
===================================================================
--- src/wp-includes/js/mce-view.js	(revision 28752)
+++ src/wp-includes/js/mce-view.js	(working copy)
@@ -719,26 +719,29 @@
 			wp.ajax.send( 'parse-embed', {
 				data: {
 					post_ID: $( '#post_ID' ).val(),
-					content: this.shortcode
+					type: self.type,
+					shortcode: this.shortcode
 				}
 			} )
-			.done( function( content ) {
+			.always( function() {
 				self.fetching = false;
-
-				if ( content.substring( 0, ( '<a href' ).length ) === '<a href' ) {
+			} )
+			.done( function( response ) {
+				if ( response ) {
+					self.parsed = response;
+					self.setHtml( response );
+				}
+			} )
+			.fail( function( response ) {
+				if ( response && response.message ) {
 					if ( self.type === 'embed' ) {
-						self.setError( self.original + ' failed to embed.', 'admin-media' );
+						self.setError( response.message, 'admin-media' );
 					} else {
-						self.setContent( self.original, null, true );
+						self.setContent( '<p>' + self.original + '</p>', null, true );
 					}
-				} else {
-					self.parsed = content;
-					self.setHtml( content );
+				} else if ( response && response.statusText ) {
+					self.setError( response.statusText, 'admin-media' );
 				}
-			} )
-			.fail( function() {
-				self.fetching = false;
-				self.setError( self.original + ' failed to embed due to a server error.', 'admin-media' );
 			} );
 		},
 		/* jshint scripturl: true */