Allow multiple Error pixels (InteractiveAdvertisingBureau#12)

vast4.xsd

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8" ?>
+<?xml version="1.0" encoding="utf-8" ?>
 <!--Created with Liquid XML Studio Developer Bundle Edition (Trial) (http://www.liquid-technologies.com)-->
 <xs:schema attributeFormDefault="unqualified"
 		   elementFormDefault="qualified"
@@ -65,7 +65,7 @@
                 name="Error"
                 type="xs:anyURI"
                 minOccurs="0"
-                maxOccurs="1">
+                maxOccurs="unbounded">
 					<xs:annotation>
 						<xs:documentation>
 							Used when there is no ad response. When the ad server does not or cannot return an Ad.
@@ -1572,4 +1572,4 @@ play as intended.
 			</xs:extension>
 		</xs:complexContent>
 	</xs:complexType>
-</xs:schema>
+</xs:schema>