app/handlers/callback: warn when invalid/missing metadata
app/handlers/callback: warn when invalid/missing metadata

Add error messages into the backend logs when the metadata is invalid
or there are missing mandatory keys.

Having errors in the backend logs is helpful when the jobs sending
callbacks are not checking the HTTP POST errors.

Signed-off-by: Kevin Hilman <f9849b02abe18c1ece6bd40346458ed53f49d31b@baylibre.com>

kernelci/kernelci-backend,kernelci/kernelci-backend