Check whether the user has permissions to add personal storage backends

Quick’n dirty back port of #8182 - master has a better fix but that
should be good enough…

missing return - OCP\JSON::success does not terminate the PHP process - which is good ;-)

Use error instead of success

Revert "Use error instead of success"

This reverts commit e2d5535a5aa436c3896e46f0b9e8ff1bd5640d4d.

Use error instead of success
Merge pull request #8261 from owncloud/stable6-backport-8182

Backport of #8182