Raise useful error for cat-log failure when listing log files

cylc/uiserver/resolvers.py

@@ -429,7 +429,7 @@ async def cat_log(cls, id_: Tokens, log, info, file=None):
             yield {'connected': False}
 
     @classmethod
-    async def cat_log_files(cls, id_: Tokens):
+    async def cat_log_files(cls, id_: Tokens, log: 'Logger') -> List[str]:
         """Calls cat log to get list of available log files.
 
         Note kept separate from the cat_log method above as this is a one off
@@ -438,22 +438,26 @@ async def cat_log_files(cls, id_: Tokens):
         file checking.
         """
         cmd: List[str] = ['cylc', 'cat-log', '-m', 'l', '-o', id_.id]
+        log.debug(f"$ {' '.join(cmd)}")
         proc_job = await asyncio.subprocess.create_subprocess_exec(
             *cmd,
             stdout=asyncio.subprocess.PIPE,
             stderr=asyncio.subprocess.PIPE,
         )
         # wait for proc to finish
-        await proc_job.wait()
+        ret_code = await proc_job.wait()
 
-        # MOTD returned in stderr, no use in returning
-        out_job, _ = await proc_job.communicate()
-        if out_job:
+        out, err = await proc_job.communicate()
+        if ret_code:
+            raise Exception(
+                f"Command failed ({ret_code}): {' '.join(cmd)}\n{err.decode()}"
+            )
+        if out:
             return sorted(
                 # return the log files in reverse sort order
                 # this means that the most recent log file rotations
                 # will be at the top of the list
-                out_job.decode().splitlines(),
+                out.decode().splitlines(),
                 reverse=True,
             )
         else:
@@ -569,7 +573,7 @@ async def query_service(
         self,
         id_: Tokens,
     ):
-        return await Services.cat_log_files(id_)
+        return await Services.cat_log_files(id_, self.log)
 
 
 def kill_process_tree(