[BUGFIX] Better error messages for FLUIDTEMPLATE object

When using FLUIDTEMPLATE with Typoscript "template.file = filename",
it will fail silently if filename does not point to a file.

Also check for template source when using Typoscript
"file = filename" in FLUIDTEMPLATE object.

Resolves: #65340
Releases: master
Change-Id: Id6aa148408cad14e49b65c0a9f4c7887cfb3fa16
Reviewed-on: https://review.typo3.org/37273


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>