[lldb] Fix minor runCmd error message formatting (llvm#110150)

This tweaks the construction of the error message when using `expect`/`runCmd`. With 
this change, the stdout/stderr is placed after the message "Command '<command>' did not 
return successfully".

Before:
```
AssertionError: False is not True : Command 'p whatever
Error output:
error: <some error message>
' did not return successfully
```

After:
```
AssertionError: False is not True : Command 'p whatever' did not return successfully
Error output:
error: <some error message>
```

Author: kastiglione

lldb/packages/Python/lldbsuite/test/lldbtest.py

@@ -172,9 +172,9 @@
 WATCHPOINT_CREATED = "Watchpoint created successfully"
 
 
-def CMD_MSG(str):
+def CMD_MSG(command):
     """A generic "Command '%s' did not return successfully" message generator."""
-    return "Command '%s' did not return successfully" % str
+    return f"Command '{command}' did not return successfully"
 
 
 def COMPLETION_MSG(str_before, str_after, completions):
@@ -990,16 +990,14 @@ def runCmd(self, cmd, msg=None, check=True, trace=False, inHistory=False):
                     print("Command '" + cmd + "' failed!", file=sbuf)
 
         if check:
+            if not msg:
+                msg = CMD_MSG(cmd)
             output = ""
             if self.res.GetOutput():
                 output += "\nCommand output:\n" + self.res.GetOutput()
             if self.res.GetError():
                 output += "\nError output:\n" + self.res.GetError()
-            if msg:
-                msg += output
-            if cmd:
-                cmd += output
-            self.assertTrue(self.res.Succeeded(), msg if (msg) else CMD_MSG(cmd))
+            self.assertTrue(self.res.Succeeded(), msg + output)
 
     def HideStdout(self):
         """Hide output to stdout from the user.