raise hy compile error on Py2.x for raise from

hy/compiler.py

@@ -680,9 +680,10 @@ class HyASTCompiler(object):
             ret += self.compile(expr.pop(0))
 
         cause = None
-        if PY3:
-            if len(expr) == 2:
-                if expr[0] == HyKeyword(":from"):
+        if len(expr) == 2 and expr[0] == HyKeyword(":from"):
+            if not PY3:
+                raise HyCompileError(
+                    "raise from only supported in python 3")
             expr.pop(0)
             cause = self.compile(expr.pop(0))
             cause = cause.expr