Make "No configured storage engine" slightly clearer by specifying why.
Summary: Inspired by T16386. While there are other reasons this could happen the likeist one is that the file size is too large.
Test Plan: Try to upload a file larger than (in the Phorge default config) 1MB, see new error.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, aklapper, Cigaryno
Differential Revision: https://we.phorge.it/D26569