improve error message for multiple documents

The YAML parser doesn't support multiple documents. This pull requests
improves the error message when the parser detects multiple YAML
documents.

src/Symfony/Component/Yaml/Parser.php

@@ -193,6 +193,11 @@ class Parser
                     }
                 }
             } else {
+                // multiple documents are not supported
+                if ('---' === $this->currentLine) {
+                    throw new ParseException('Multiple documents are not supported.');
+                }
+
                 // 1-liner optionally followed by newline
                 $lineCount = count($this->lines);
                 if (1 === $lineCount || (2 === $lineCount && empty($this->lines[1]))) {

src/Symfony/Component/Yaml/Tests/ParserTest.php

@@ -482,6 +482,27 @@ EOF;
         $this->parser->parse($yaml);
     }
 
+    /**
+     * @expectedException \Symfony\Component\Yaml\Exception\ParseException
+     * @expectedExceptionMessage Multiple documents are not supported.
+     */
+    public function testMultipleDocumentsNotSupportedException()
+    {
+        Yaml::parse(<<<EOL
+# Ranking of 1998 home runs
+---
+- Mark McGwire
+- Sammy Sosa
+- Ken Griffey
+
+# Team ranking
+---
+- Chicago Cubs
+- St Louis Cardinals
+EOL
+        );
+    }
+
     /**
      * @expectedException \Symfony\Component\Yaml\Exception\ParseException
      */