[FEATURE] Special handling for log records with "exception" keys

[cweiske]

TYPO3 scheduler catches exceptions of failed tasks and logs messages with the exception as "exception" data property.

PHP Exception objects do not get serialized by json_encode() and thus never reach Sentry; all information about them is lost.

If a log record contains an 'exception' data key, then the exception itself is logged to Sentry. The log message is kept as breadcrumb.

Resolves: https://github.com/networkteam/sentry_client/issues/106