[BUGFIX] Extension Manager Cleanup

Beside some type hintings and indent correction there is now a check
in response for timeout with fetching translation, so the UI is not
blocked anymore if a timeout occurs.

Change-Id: I670a2df4fddab0be6b0cb05e69e009c08db5e1fa
Resolves: #28667
Reviewed-on: http://review.typo3.org/4012
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn