Fix warnings in case of incorrect data

[vkhramtsov]

If somebody pass incorrect input array to format, php will raise several notices, because no any checks performed before access to array indexes.
Please review.

[Seldaek]

I think either the code needs to handle missing keys like the LogstashFormatter does or you just should not call it with invalid records, but rejecting messages silently is not good.

[vkhramtsov]

In our code we perform checks that value returned from formatter is instance of Message.
Anyway, I can throw an Exception in case of invalid data.
About silently ignore invalid data: your code actually does it in \Monolog\Formatter\NormalizerFormatter::normalize. From my point of view, code should not raise warnings or notices in any case, even if called with incorrect parameters.

[Seldaek]

I don't see where the normalizer ignores data. Anyway I would be ok to throwing an exception if some essential keys are needed (or if they are not essential, then do a isset check and go through with a default value or null when they're missing), but the current PR is not ok for me.

[mimmi20]

Could you add a DocBlock here?

[vkhramtsov]

Added

[vkhramtsov]

Updated, please review.

[vkhramtsov]

@Seldaek, what do you think about this pr?

[Seldaek]

I fixed it in another way so it mostly works except if essential data is missing then you get an exception.