dvc: check stage name before running command

Closes #2150.

dvc/stage.py

@@ -480,6 +480,8 @@ class Stage(object):
 
         if not fname:
             fname = Stage._stage_fname(stage.outs, add=add)
+        stage._check_dvc_filename(fname)
+
         wdir = os.path.abspath(wdir)
 
         if cwd is not None:

tests/func/test_run.py

@@ -97,16 +97,6 @@ class TestRunMissingDep(TestDvc):
 
 class TestRunBadStageFilename(TestDvc):
     def test(self):
-        with self.assertRaises(StageFileBadNameError):
-            self.dvc.run(
-                cmd="",
-                deps=[],
-                outs=[],
-                outs_no_cache=[],
-                fname="empty",
-                cwd=os.curdir,
-            )
-
         with self.assertRaises(StageFileBadNameError):
             self.dvc.run(
                 cmd="",
@@ -1000,3 +990,17 @@ class TestPersistentOutput(TestDvc):
         # it should run the command again, as it is "ignoring build cache"
         with open("greetings", "r") as fobj:
             assert "hello\nhello\n" == fobj.read()
+
+
+def test_bad_stage_fname(repo_dir, dvc_repo):
+    dvc_repo.add(repo_dir.FOO)
+    with pytest.raises(StageFileBadNameError):
+        dvc_repo.run(
+            cmd="python {} {} {}".format(repo_dir.CODE, repo_dir.FOO, "out"),
+            deps=[repo_dir.FOO, repo_dir.CODE],
+            outs=["out"],
+            fname="out_stage",  # Bad name, should end with .dvc
+        )
+
+    # Check that command hasn't been run
+    assert not os.path.exists("out")