From 64baccbecc478d7cbaa06c35b81cb13962cb6292 Mon Sep 17 00:00:00 2001
From: Tim Pillinger <26465611+wxtim@users.noreply.github.com>
Date: Mon, 15 May 2023 08:57:33 +0100
Subject: [PATCH] Update resolvers.py

---
 cylc/uiserver/resolvers.py | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/cylc/uiserver/resolvers.py b/cylc/uiserver/resolvers.py
index f9a8574c..04e52f03 100644
--- a/cylc/uiserver/resolvers.py
+++ b/cylc/uiserver/resolvers.py
@@ -317,19 +317,14 @@ async def play(cls, workflows, args, workflows_mgr, log):
                 if ret:
                     # command failed
                     out, err = proc.communicate()
-                    if 'previously run with' in out:
-                        # Can't upgrade:
-                        msg = out
-                    else:
-                        msg = (
-                            f'Could not start {tokens["workflow"]}'
-                            f' - {cmd_repr}'
-                        )
-
+                    msg = out.strip() or err.strip() or (
+                        f'Could not start {tokens["workflow"]}'
+                        f' - {cmd_repr}'
+                    )
                     raise Exception(
                         msg
-                        # suppress traceback unless in debug mode
-                        + (f' - {err}' if DEBUG else '')
+                        # include possible traceback in debug mode
+                        + (f' - {err}' if DEBUG and not out.strip() else '')
                     )
 
             except Exception as exc: