Update error message when no agents are available during workspace creation

Issue: #470450

What does this MR do and why?

• Updates message that is displayed when a workspace cannot be created due to unavailable cluster agents. Code has been updated for two entrypoints
  • when a user tries to create a workspace from a specific project
  • when a user tries to create a workspace directly from the Workspaces page
• Specs have been updated to verify the introduced changes

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