ybd should error out in the case of missing morph files

[gtristan]

Newly added xdg-app-common.morph refers to inexistant strata/xdg-app-common/xdg-app.morph

But instead of letting us know about this; we just build... build output does not print anything about the missing morph file:

1 15-10-29 00:00:10 [1/2/175] [xdg-app] Starting build of xdg-app.0179839578248c56d87022243683e894de3d7008cc46f8042b1688726865310d 
0 15-10-29 00:00:14 [xdg-app] This is already building, so wait and retry 
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Git checkout upstream:xdg-app in /home/tristan/ybd/tmp/tmpLrHUvp/xdg-app.build 
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Upstream version aa535b7b 0.4.4-1-gaa535b7 (0.4.4 + 1 commits) 
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Git submodules 
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Git checkout upstream:libglnx in /home/tristan/ybd/tmp/tmpLrHUvp/xdg-app.build/libglnx 
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Upstream version 7310bbdb (No tag found) 
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Autodetected build system is autotools
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Logging build commands to /home/tristan/ybd/artifacts/xdg-app.0179839578248c56d87022243683e894de3d7008cc46f8042b1688726865310d.build-log.1 
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Running configure-commands
1 15-10-29 00:00:14 [1/2/175] [xdg-app] Running command:
... and then the error here ...