fix(dap): confusing error when debug adapter not found

[winter-loo]

According to the neotest documentation, the strategy should be a table or a function. Current implementation returns a string as the straetegy value in case of failure. See image below in case I forgot to install the codelldb debug adapter.

This commit fixes the implementation and refines the error message.
After this fix, the user will see the error message as:

when the debug adapter is not installed.

[github-actions]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(lsp): some lsp-related bugfix

• Pull request title has the appropriate conventional commit prefix.

If applicable:

• Tested
  • Tests have been added.
  • Tested manually (Steps to reproduce in PR description).
• Updated documentation.

[mrcjkb]

Thanks 🙏