Symfony version(s) affected: >=3.4.37

Description
The following error might appear which blocks Debug Toolbar to be loaded:

Uncaught PHP Exception ErrorException: "Notice: Undefined index: errorCount" at /Users/torinaki/www/htdocs/fithealthy2/vendor/symfony/http-kernel/DataCollector/LoggerDataCollector.php line 190

How to reproduce
TBD

@trigger_error("1", E_USER_DEPRECATED)

Such a log message might be thrown by:
https://github.com/symfony/symfony/blob/3.4/src/Symfony/Component/Config/Definition/ArrayNode.php#L238
in case if configuration child is deprecated and depreciation message for some reason is "1"

Possible Solution
Fix logic of:

1. There no guarantee that "errorCount" exists but it is not a root cause.
   

   symfony/src/Symfony/Component/HttpKernel/DataCollector/LoggerDataCollector.php

   Line 195 in e60a876

   $sanitizedLogs[$message]['errorCount'] += $exception->count;

2. $sanitizedLogs contains numeric and assoc. keys. It should be avoided:
   

   symfony/src/Symfony/Component/HttpKernel/DataCollector/LoggerDataCollector.php

   Line 175 in e60a876

   $sanitizedLogs[] = $log;

   
   

   symfony/src/Symfony/Component/HttpKernel/DataCollector/LoggerDataCollector.php

   Line 195 in e60a876

   $sanitizedLogs[$message]['errorCount'] += $exception->count;

Additional Context
It happened because of incorrect passed deprecation message:
https://github.com/Sylius/SyliusMailerBundle/blob/1.4/src/Bundle/DependencyInjection/Configuration.php#L76
It was possible because of missing type hint:
https://github.com/symfony/symfony/blob/3.4/src/Symfony/Component/Config/Definition/BaseNode.php#L170
which is fixed in 5.0 version of Symfony:
https://github.com/symfony/symfony/blob/5.0/src/Symfony/Component/Config/Definition/BaseNode.php#L204