Stop hiding critical debug info in helpers (#988) (#997)

* [Resolve #988] Stop hiding debug info in helpers

Before this, the catch_exceptions function in cli/helpers would catch a
range of exceptions and then hide all but the error message from the
caller.

Over the years, this has caused myself and others quite a lot of lost
time, as it is now often quite unclear what caused Sceptre to fail.

Simply re-raising the original exception provides valuable information
to allow users of Sceptre to debug their failing code.

* Refactor according to Jon's suggestion

* Refactor to implement Craig Hurley's suggestion

* Restore master version of tests

* Add unit tests

sceptre/cli/helpers.py

@@ -27,16 +27,23 @@ def catch_exceptions(func):
         simplified.
     :returns: The decorated function.
     """
+    def logging_level():
+        logger = logging.getLogger(__name__)
+        return logger.getEffectiveLevel()
+
     @wraps(func)
     def decorated(*args, **kwargs):
         """
         Invokes ``func``, catches expected errors, prints the error message and
-        exits sceptre with a non-zero exit code.
+        exits sceptre with a non-zero exit code. In debug mode, the original
+        exception is re-raised to assist debugging.
         """
         try:
             return func(*args, **kwargs)
         except (SceptreException, BotoCoreError, ClientError, Boto3Error,
                 TemplateError) as error:
+            if logging_level() == logging.DEBUG:
+                raise
             write(error)
             sys.exit(1)
 

tests/test_cli.py

@@ -56,14 +56,25 @@ def teardown_method(self, test_method):
         self.patcher_StackActions.stop()
 
     @patch("sys.exit")
-    def test_catch_excecptions(self, mock_exit):
+    def test_catch_exceptions(self, mock_exit):
         @catch_exceptions
         def raises_exception():
             raise SceptreException()
 
         raises_exception()
         mock_exit.assert_called_once_with(1)
 
+    def test_catch_exceptions_debug_mode(self):
+        @catch_exceptions
+        def raises_exception():
+            raise SceptreException()
+
+        logger = logging.getLogger("sceptre")
+        logger.setLevel(logging.DEBUG)
+
+        with pytest.raises(SceptreException):
+            raises_exception()
+
     @pytest.mark.parametrize("command,files,output", [
         # one --var option
         (