[BUGFIX] Use recursive option of mkdir in t3lib_div::mkdir_deep

Additionally throw exceptions instead of a return string if directory could
not be created.

Change-Id: I90da96e1b62d28997573d821c62b7b24b3ef2ba3
Resolves: #26240
Depends: #26230
Releases: 4.6
Reviewed-on: http://review.typo3.org/1716
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog