wrap SyntaxError with \exception{}

Doc/ref/ref4.tex

@@ -127,7 +127,7 @@ to delete the name.  An error will be reported at compile time.
 
 If the wild card form of import --- \samp{import *} --- is used in a
 function and the function contains or is a nested block with free
-variables, the compiler will raise a SyntaxError.
+variables, the compiler will raise a \exception{SyntaxError}.
 
 If \keyword{exec} is used in a function and the function contains or
 is a nested block with free variables, the compiler will raise a