Adapt Forbidden Exception's "s:message" on blacklisted files

[SamuAlfageme]

When trying to upload .htaccess files; which the server does not support (since they're part of blacklisted_files in the capabilities https://github.com/owncloud/core/blob/v10.0.4/apps/files/lib/Capabilities.php#L62).

Client version 2.4 removed .htaccess from its fixed exclusion list (owncloud/client#5748); users might still manually add those there (and maybe client should have different exclusion lists for each capabilities' blacklisted_files from the servers it is connected to).

But still, server should not reply 507 "Insufficient Storage" upon blacklisted_files upload. A 4XX error with a more user-friendly message (#29948) would be handy.

[ownclouders]

GitMate.io thinks the contributor most likely able to help you is @PVince81.

[mmattel]

Not tested but maybe related, could be the same behaviour if you try with exluded_dirctories.
Both blacklisted and excluded should return imho either 403 - Forbidden or maybe 420 - Policy Not Fulfilled

[SamuAlfageme]

As @ckamm points out in owncloud/client#6302 (comment) 507 happens in the context of 9.X - therefore adjusting the label.

Also leaving this open in the context of improving the error message/sub-code (#29948) for better troubleshooting and UX in the clients: