Improve error handling

[praseodym]

Events current emails information about exceptions without logging them:

events/src/main/java/ch/wisv/events/core/service/mail/MailServiceImpl.java

Lines 110 to 122 in 6916f8f

	/**
	* Method mails about a error.
	*
	* @param subject of type String
	* @param exception of type Exception
	*/
	@Override
	public void sendError(String subject, Exception exception) {
	final Context ctx = new Context(new Locale("en"));
	ctx.setVariable("exception", exception);
	this.sendMailWithContent("[email protected]", subject, this.templateEngine.process("mail/error", ctx));
	}

These emails are either broken or not read by the current developers. It would be better to log exceptions to stderr or use Google Cloud Error Reporting (with the GKE integration or Java client library).