Improve the error handling for towncrier

The changelog shows up via stdout, everything else via stderr. We dump as much information as we can into the changelog to make debugging errors easier.

scripts/templating/matrix_templates/units.py

@@ -839,21 +839,28 @@ class MatrixUnits(Units):
             tc_lines = []
             if os.path.isdir(tc_path):
                 logger.info("Generating towncrier changelog for: %s" % name)
-                try:
-                    raw_log = subprocess.check_output(
+                p = subprocess.run(
                     ['towncrier', '--version', 'Unreleased Changes', '--name', name, '--draft'],
-                        stderr=subprocess.PIPE,
                     cwd=tc_path,
-                    ).strip().decode('UTF-8')
+                    check=False, # We'll manually check the exit code
+                    stderr=subprocess.PIPE,
+                    stdout=subprocess.PIPE,
+                )
+                if p.returncode != 0:
+                    # Something broke - dump as much information as we can
+                    logger.error("Towncrier exited with code %s" % p.returncode)
+                    logger.error(p.stdout.decode('UTF-8'))
+                    logger.error(p.stderr.decode('UTF-8'))
+                    raw_log = ""
+                else:
+                    raw_log = p.stdout.decode('UTF-8')
 
                     # This is a bit of a hack, but it does mean that the log at least gets *something*
                     # to tell us it broke
                     if not raw_log.startswith("Unreleased Changes"):
+                        logger.error("Towncrier appears to have failed to generate a changelog")
                         logger.error(raw_log)
                         raw_log = ""
-                except subprocess.CalledProcessError as e:
-                    logger.error(e)
-                    raw_log = ""
                 tc_lines = raw_log.splitlines()
 
             title_part = None