Improving message description for compile error in get_fixtu…

[AlejandroBlanco2001]

resolves dbt-labs/dbt-core#10014

Problem

When running SQL unit tests, when a relation does not exist (or the model does not exist), a compilation error is thrown which does not give any idea why this is happening.

Solution

Therefore, a small new feature is added where the error message is improved to give a better idea on how to resolve the error when running the test.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

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[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[AlejandroBlanco2001]

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In order for us to review and merge your code, please submit the Individual Contributor License Agreement form attached above above. If you have questions about the CLA, or if you believe you've received this message in error, please reach out through a comment on this PR.

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[jtcohen6]

looks good!

note: this is simply an update to the language in an exception we're raising, to provide the end user with greater clarity on the error :)