[BUGFIX] Use correct array structure for EM error messages

The extension manager stores errors (like exceptions that
are thrown while installing an extension) in an array and
passes them to a fluid template.

The array has to be in a defined structure like
$errorMessages[extension name][i] = array(
»       error code,
»       error message
)

Currently, if an extension is installed from ter and an error occurs
while downloading the extension, the code and text are stored directly
under the extension name and therefore the array is not correctly parsed
by the fluid template.

Resolves: #59991
Releases: 6.3, 6.2
Change-Id: Id330f0af2a99ab99cb8b736332cb8506c857716c
Reviewed-on: https://review.typo3.org/31267
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters