Improved error output on PY3

Chaining of errors using "raise from" via future's wrapper was losing a
lot of debugging information, including traceback for the cause of the
error. Instead, we now use the real "raise from" on PY3 and only use
future's replication of it on PY2. This means that there'll be less good
debugging info on PY2, but that's not a priority anyway.

src/python/pimlico/core/modules/execute.py

@@ -11,7 +11,8 @@
 from __future__ import unicode_literals
 
 from future import standard_library
-from future.utils import raise_from
+
+from pimlico.utils.core import raise_from
 
 standard_library.install_aliases()
 from builtins import str
@@ -23,7 +24,7 @@
 from io import StringIO
 from tarfile import TarFile
 from textwrap import wrap
-from traceback import format_exc, format_tb
+from traceback import format_exc, format_tb, format_exception
 
 from datetime import datetime
 

src/python/pimlico/utils/core.py

@@ -6,6 +6,8 @@
 from builtins import object
 from contextlib import contextmanager
 
+from future.utils import raise_from as future_raise_from, PY3
+
 import sys
 import ast
 
@@ -220,3 +222,19 @@ def __get__(self, obj, cls):
             return self
         value = obj.__dict__[self.func.__name__] = self.func(obj)
         return value
+
+
+def raise_from(exc, cause):
+    """
+    Like future's raise_from function. However, on Py3, just calls raise X from Y.
+    On Py2, defers to future's replacement.
+
+    This means that we get the full functionality of raise from on PY3, which
+    is our main priority. If you run on PY2, you get less debugging information
+    in some cases, but that's no reason to ruin the debugging information on PY3 too!
+
+    """
+    if PY3:
+        raise exc from cause
+    else:
+        future_raise_from(exc, cause)