Make DboSource::lastError() fallback to the connection.

If there is no argument the connection error should be checked
for an error.
Add a test for most of lastError().  Mocking PDO is a pain.
Fixes #2046

lib/Cake/Model/Datasource/DboSource.php

@@ -460,7 +460,11 @@ class DboSource extends DataSource {
  * @return string Error message with error number
  */
 	public function lastError(PDOStatement $query = null) {
-		$error = $query->errorInfo();
+		if ($query) {
+			$error = $query->errorInfo();
+		} else {
+			$error = $this->_connection->errorInfo();
+		}
 		if (empty($error[2])) {
 			return null;
 		}

lib/Cake/Test/Case/Model/Datasource/DboSourceTest.php

@@ -39,6 +39,10 @@ class DboTestSource extends DboSource {
 	public function setConfig($config) {
 		$this->config = $config;
 	}
+
+	public function setConnection($conn) {
+		$this->_connection = $conn;
+	}
 }
 
 /**
@@ -786,4 +790,20 @@ class DboSourceTest extends CakeTestCase {
 		$this->assertEqual(' GROUP BY created', $result);
 	}
 
+/**
+ * Test getting the last error.
+ */
+	function testLastError() {
+		$result = $this->db->lastError();
+		$this->assertNull($result);
+
+		$stmt = $this->getMock('PDOStatement');
+		$stmt->expects($this->any())
+			->method('errorInfo')
+			->will($this->returnValue(array('', 'something', 'bad')));
+
+		$result = $this->db->lastError($stmt);
+		$expected = 'something: bad';
+		$this->assertEquals($expected, $result);
+	}
 }