[BUGFIX] EM: Mask overlay stays active after error message

If you try to install a new extension from TER which is incompatible
with the current TYPO3 version, an error is shown but the loading mask
isn't removed. This patch unmask the overlay to be able to work again.

Resolves: #53460
Releases: 6.2
Change-Id: Ibf9a31de6a8f56715e20ba5b5f2c49e555b16e12
Reviewed-on: https://review.typo3.org/25309
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Steffen Müller
Tested-by: Steffen Müller