[Backport to main] tune error message for no ML node case

[ylwu-amzn]

Description

[Describe what this change achieves]

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov-commenter]

Codecov Report

Merging #780 (29df28c) into main (56cdb85) will decrease coverage by 0.33%.
The diff coverage is 0.00%.

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@             Coverage Diff              @@
##               main     #780      +/-   ##
============================================
- Coverage     84.17%   83.85%   -0.33%     
+ Complexity     1039     1035       -4     
============================================
  Files            99       99              
  Lines          3740     3740              
  Branches        344      344              
============================================
- Hits           3148     3136      -12     
- Misses          445      456      +11     
- Partials        147      148       +1     

Flag	Coverage Δ
ml-commons	83.85% <0.00%> (-0.33%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../java/org/opensearch/ml/task/MLTaskDispatcher.java	32.94% <0.00%> (ø)
.../cluster/MLCommonsClusterManagerEventListener.java	65.62% <0.00%> (-12.50%)	⬇️
...ain/java/org/opensearch/ml/task/MLTaskManager.java	61.92% <0.00%> (-4.07%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.