[BUGFIX] Extension Upload ends in PHP Error

Adds an additional check after processing of the ext_emconf.php, throwing
an exception if EmConfUtility::includeEmConf returns FALSE.

Change-Id: Ib1404e2911b8372c5d8ff99aaf2e6313eaf7e157
Fixes: #53210
Releases: 6.2
Reviewed-on: https://review.typo3.org/26741
Reviewed-by: Markus Klein
Tested-by: Markus Klein