You can still use $context here.
Something like {query}; had {warning_count} warnings:

Please move this out of the has_failed() check

Separate review please :)

Minor optimization:

$warnings = $query->warnings();


and then:

if ($warnings !== NULL)...
foreach ($warnings as ...
count($warnings)...

I don't think putting the warnings into context is the most elegant solution here.

At the very least, every warning should appear on a new line, so not separated by ;.

Now we also need a test for warnings on error :)

You can already define the {query}; had ... string   here. Saves a string concat later