[server] print backend exception if we fail to execute an sql script
It's more useful to see
could not open extension control file "/usr/share/postgresql/9.4/extension/postgis.control": No such file or directory
rather than merely
-> ERROR, skipping /usr/share/cubicweb/cubes/postgis/schema/_ext.postgres.sql
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*