hdl.dsl: clarify error message for incorrect nesting.

Refs #133.

nmigen/hdl/dsl.py

@@ -133,7 +133,7 @@ class Module(_ModuleBuilderRoot, Elaboratable):
                 raise SyntaxError("{} is not permitted outside of {}"
                                   .format(construct, context))
             else:
-                raise SyntaxError("{} is not permitted inside of {}"
+                raise SyntaxError("{} is not permitted directly inside of {}"
                                   .format(construct, self._ctrl_context))
 
     def _get_ctrl(self, name):

nmigen/test/test_hdl_dsl.py

@@ -345,7 +345,7 @@ class DSLTestCase(FHDLTestCase):
         m = Module()
         with m.Switch(self.s1):
             with self.assertRaises(SyntaxError,
-                    msg="If is not permitted inside of Switch"):
+                    msg="If is not permitted directly inside of Switch"):
                 with m.If(self.s2):
                     pass
 
@@ -480,6 +480,16 @@ class DSLTestCase(FHDLTestCase):
             with m.FSM():
                 m.next = "FOO"
 
+    def test_If_inside_FSM_wrong(self):
+        m = Module()
+        with m.FSM():
+            with m.State("FOO"):
+                pass
+            with self.assertRaises(SyntaxError,
+                    msg="If is not permitted directly inside of FSM"):
+                with m.If(self.s2):
+                    pass
+
     def test_auto_pop_ctrl(self):
         m = Module()
         with m.If(self.w1):