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

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

@@ -30,14 +30,14 @@ 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