[Resolve Sceptre#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.

sceptre/cli/helpers.py

@@ -1,5 +1,4 @@
 import logging
-import sys
 from functools import partial, wraps
 
 import json
@@ -30,14 +29,14 @@ def catch_exceptions(func):
     def decorated(*args, **kwargs):
         """
         Invokes ``func``, catches expected errors, prints the error message and
-        exits sceptre with a non-zero exit code.
+        re-raises.
         """
         try:
             return func(*args, **kwargs)
         except (SceptreException, BotoCoreError, ClientError, Boto3Error,
                 TemplateError) as error:
             write(error)
-            sys.exit(1)
+            raise
 
     return decorated
 

tests/test_cli.py

@@ -55,14 +55,13 @@ def teardown_method(self, test_method):
         self.patcher_ConfigReader.stop()
         self.patcher_StackActions.stop()
 
-    @patch("sys.exit")
-    def test_catch_excecptions(self, mock_exit):
+    def test_catch_excecptions(self):
         @catch_exceptions
         def raises_exception():
             raise SceptreException()
 
-        raises_exception()
-        mock_exit.assert_called_once_with(1)
+        with pytest.raises(SceptreException):
+            raises_exception()
 
     @pytest.mark.parametrize("command,files,output", [
         # one --var option