Removed deprecated error classes and added class_alias for them instead in deprecated.php

Author: vladar

src/Error.php

@@ -4,6 +4,10 @@
 }
 
 // Renamed as of 8.0:
+class_alias('GraphQL\Error\Error', 'GraphQL\Error');
+class_alias('GraphQL\Error\FormattedError', 'GraphQL\FormattedError');
+class_alias('GraphQL\Error\SyntaxError', 'GraphQL\SyntaxError');
+
 class_alias('GraphQL\Language\AST\ArgumentNode', 'GraphQL\Language\AST\Argument');
 class_alias('GraphQL\Language\AST\BooleanValueNode', 'GraphQL\Language\AST\BooleanValue');
 class_alias('GraphQL\Language\AST\DefinitionNode', 'GraphQL\Language\AST\Definition');