There are multiple duplicate reports of errors during template rendering
due to broken custom templates.
Unfortunately the error returned here is somewhat difficult for users to
understand and it doesn't return the context of the error.
This PR attempts to parse the error returned by the template renderer to
add in some further context including the filename of the template AND
the preceding lines within that template file.
Ref #23274
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Signed-off-by: Andrew Thornton <art27@cantab.net>
8f3aaef374