fix(dbless): handle unexpected types when flattening errors

[flrgh]

This is a bugfix backport of #10896.

[flrgh]

argh, not ready yet (git is hard)

[flrgh]

note: changes since last approval were just to fix a test/assertion compatibility thing. I squashed it into the first commit (because that's where it should go, but here's the diff:

diff --git spec/02-integration/04-admin_api/15-off_spec.lua spec/02-integration/04-admin_api/15-off_spec.lua
index fd99d1d5bd..e7fe91a9fe 100644
--- spec/02-integration/04-admin_api/15-off_spec.lua
+++ spec/02-integration/04-admin_api/15-off_spec.lua
@@ -1096,11 +1096,11 @@ describe("Admin API #off /config [flattened errors]", function()
       helpers.intercept(body)
     end

-    assert.logfile().has.no.line("[emerg]", true, 0)
-    assert.logfile().has.no.line("[crit]",  true, 0)
-    assert.logfile().has.no.line("[alert]", true, 0)
-    assert.logfile().has.no.line("[error]", true, 0)
-    assert.logfile().has.no.line("[warn]",  true, 0)
+    assert.logfile().has.no.line("[emerg]", true, 0.01)
+    assert.logfile().has.no.line("[crit]",  true, 0.01)
+    assert.logfile().has.no.line("[alert]", true, 0.01)
+    assert.logfile().has.no.line("[error]", true, 0.01)
+    assert.logfile().has.no.line("[warn]",  true, 0.01)

     return errors
   end