[UPDATE] Better error handling for the IncludeViewhelper

If the Package/Controller is unknown the whole site will just show a big TYPO3.Flow
error message. This patch introduces the viewhelper argument "exceptionAs", this will make
it possible to display a custom error message in place of the plugin.

<mdc:include package="Kollode.TestPlugi" controller="Standard" action="index" exceptionAs="exception">
	<div data-alert class="alert-box alert">{exception.message}</div>
</mdc:include>
[FEATURE] Configure a custom 404 page

At the moment the user would just get a ugly 404 error, if
the the requested page dosen't exists. This patch makes it
possible to configure a fallback page that should be used
and only display the ugly 404 error if the fallback page is not
configured or dosen't exists either.

Scriptme:
  MarkdownContent:
    errorHandling:
      404: '404'
Merge pull request #3 from Kollode/errorHandling

[UPDATE] Better error handling for the IncludeViewhelper