Swap to skip file if not found

[alanmcruickshank]

While testing 1.0.0 I found this quality of life bug.

While using the dbt templater, if you're not careful with the path specification you could accidentally specify files explicitly in your dependencies which aren't actually rendered (as I found out in brooklyn-data/dbt_artifacts#125). I don't think it's a bug but it does currently result in a fatal RuntimeError if sqlfluff tries to lint a file which isn't found.

I think this is the wrong behaviour and while we should still warn the user, the linting of the rest of the project should continue. Hence changing the error to SQLTemplaterSkipFile instead of RuntimeError.

[tunetheweb]

Approved with one question

[codecov]

Codecov Report

Merging #3464 (2005859) into main (8aa74a4) will not change coverage.
The diff coverage is n/a.

❗ Current head 2005859 differs from pull request most recent head 25a42b1. Consider uploading reports for the commit 25a42b1 to get more accurate results

@@            Coverage Diff            @@
##              main     #3464   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          171       171           
  Lines        12898     12898           
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  Hits         12898     12898           

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